Image forming apparatus

ABSTRACT

An image forming apparatus for forming color images comprising at least two or more colors, having a function of magnification correction of image size by one page unit, the image forming apparatus including: an image carrier; a polygonal mirror rotator independently provided for each color; and a controller which simultaneously conducts first control for changing rotation speed of the polygonal mirror rotator in order for changing image size in a sub-scanning direction perpendicular to a main scanning direction, and second control for correcting a correction amount for color registration error depending on magnification correction of image size, and for adjusting a rotating phase of the polygonal mirror rotator depending on the corrected correction amount for color registration error.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on Japanese Patent Application No.2005-274553 filed with Japan Patent Office on Sep. 21, 2005, the entirecontent of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an image forming apparatus that ispreferably applied to a black-and white or color digital multifunctionalmachine equipped with copying functions, facsimile functions and printerfunctions and to a copier.

2. Description of Related Art

In recent years, there has come to be put to practical use a digitalcolor copier that conducts color image forming based on color image datarelating to red (R) color, green (G) color and blue (B) color acquiredfrom colored document images. In the copier of this kind, imageinformation of the document is read by a scanner, and color image datarelating to image information of the document are acquired.

Further, a laser recording apparatus is mounted on the copier, and alaser beam emitted from a semiconductor laser light source is used forexposure scanning on a photoreceptor drum having thereon prescribedvoltage to record images, depending on YMCK image data which areobtained by color-converting RGB image data acquired from a scanner intoimage data of yellow (Y) color, magenta (M) color, cyan (C) color andblack (K) color. Images recorded on the photoreceptor drum are developedby each toner of each color, then, colors are superposed on anintermediate transfer body, for example, and each image is transferredonto a prescribed sheet from the intermediate transfer body, to befixed. As a result, a color document image can be copied.

In a field of the color image forming apparatus of this kind, anapparatus wherein a color image can be formed on each of both sides ofthe sheet has been developed and is manufactured. Double-face formingfunctions are used for forming an image for a front cover on a sheet andfor forming an image for a back cover on a sheet, when creating abooklet, for example. In many cases, a sheet that is thicker than asheet mentioned in the text is used as a sheet for each of the frontcover and back cover.

Sheets for the front cover and the back cover after double-face imageforming are supposed to be subjected to post-processing such ascenter-folding and staple processing. In the double-face image formingprocessing of this kind, it is known that, after an image is formed onone side of a sheet, the sheet shrinks. The reason for this phenomenonis that a sheet onto which a color toner image has been transferred issubjected to thermal shrinkage by fixing processing, and the thicker thesheet is, the more remarkable the shrinkage is.

Each of FIGS. 17(A) and 17(B) is a diagram illustrating an example ofshrinkage of sheet size in the case of double-face image forming. SheetP shown in FIG. 17 (A) is in the state before fixing after beingsubjected to secondary transfer of color toner images. In sheet sizesfor sheet P, a longitudinal length is L mm, and a lateral width is W mm.Sheet P′ shown in FIG. 17(B) is in the state after fixing of sheet P. Insheet sizes for sheet P′, a longitudinal length is constricted to L′ mm,and a lateral width is constricted to W′ mm. The reason for shrinkage ofsheet sizes is considered to be moisture dessipation in the course offixing. An image needs to be formed on the rear face of the sheet,taking such shrinkage of sheet sizes of sheet P into consideration.Incidentally, if image forming conditions are not adjusted to sheetsizes L′ mm×W′ mm after shrinkage, an image forming position (size) forthe front face is deviated from that for the rear face.

A driving clock (hereinafter referred to as CLK) frequency of a polygonmotor is changed, taking such shrinkage of sheet sizes of sheet P intoconsideration. When F0 represents polygon driving CLK frequency beforeshrinkage, namely, in the course of image forming on the front face, andF represents polygon driving CLK frequency after shrinkage, namely, inthe course of image forming on the rear face, establishment is made sothat F=F0×L/L′ may hold.

Further, pixel CLK frequency that controls a laser beam is changed. Whenf0 represents pixel CLK frequency before shrinkage and f representspixel CLK frequency after shrinkage, establishment is made so thatf=(L/L′)×(W/W′)×f0 may hold. By changing a polygon driving CLK frequencyand a pixel CLK frequency, in consideration of shrinkage in sheet sizesfor sheet P as stated above, it is possible to obtain images which arewell-registered between the front face and the rear face.

Further, when the polygon driving CLK frequency is changed from F0 to Funder the assumption that V0 represents a process linear speed beforeshrinkage, G0 represents a gap between processes before shrinkage,process gap G represents a distance between units and V represents aprocess linear speed, apparent process linear speed V is changed asshown below.

-   (1) Apparent process linear speed V=V0×F0/F=v0×L′/L-   (2) Gap between processes G (pixel)=G0×V0/V=G0×L/L′    In this case, the process linear speed V corresponds to a rotation    speed of a photoreceptor representing an image carrier on which an    image is formed.

Therefore, correction for an amount of front-face/rear-facemagnification change (which is also called front-face/rear-facemagnification correction or image size correction, after this) is neededeven for a correction amount for color registration error whichcorresponds to gap between processes G. Accordingly, a polygon mirrorwhich requires plane phase adjustment is subjected to practice of planephase control when switching between front face and rear face. Controlof the rotation speed of the polygon mirror and control of plane phaseof a polygon mirror for each color of Y, M and C are practiced not onlyfor double-face image forming processing but also for switching oftrays.

For practicing image size correction in the case of switching betweenthe front face and the rear face of a sheet or between trays, there isemployed a method to control a rotation speed and a phase of a polygonmirror. Each of FIGS. 18(A)-18(I) is a time chart showing an example ofimage forming operations (for Y color) in the case of switching trays inan image writing unit for each of Y, M, C and K, relating to theconventional example.

A VTOP signal shown in FIG. 18(A) is a signal that rises insynchronization with an index signal (hereinafter referred to as KIDXsignal) for forming K color images shown in FIG. 18(I), after a leadingedge of the sheet fed out of tray 1 is detected by an unillustratedleading edge detection sensor. YVV start timing shown in FIG. 18(B) isfor a signal that rises in synchronization with KIDX signal, where anunillustrated KIDX counter is started, and the number of pulses for KIDXsignal is counted.

A YVV signal shown in FIG. 18(C) is a signal that rises insynchronization with an index signal (hereinafter referred to as YIDXsignal) for forming Y color images shown in FIG. 18(D). During theperiod of “H” level of the YVV signal, an image in Y color is formed ona sheet coming from tray 1, and after completion of the foregoing, thereis made control for changing a rotation speed of a polygon mirror forforming an image in Y color. In this case, a frequency of the YIDXsignal is fluctuated-until the rotation speed of the polygon mirror isstabilized. With regard to the sheet for second page fed out of tray 2,image forming for Y color is started after waiting for stabilizing timeTy1 during which a rotation of the polygon mirror is stabilized.

In the same way, during the period of “H” level of the MVV signal shownin FIG. 18(E), an image in M color is formed on a sheet coming from tray1, and after completion of the foregoing, there is made control forchanging a rotation speed of a polygon mirror for forming-an image in Mcolor. In this case, a frequency of the MIDX signal is fluctuated untilthe rotation speed of the polygon mirror for M color is stabilized.Phase change is controlled after waiting for stabilizing time Tm1 duringwhich a rotation of the polygon mirror is stabilized. With regard to thesheet for second page fed out of tray 2, image forming for M color isstarted after waiting for stabilizing time Tm2 during which a rotationof the polygon mirror for M color is stabilized.

Further, during the period of “H” level of the CVV signal shown in FIG.18(F), an image in C color is formed on a sheet coming from tray 1, andafter completion of the foregoing, there is made control for changing arotation speed of a polygon mirror for forming an image in C color. Inthis case, a frequency of the CIDX signal is fluctuated until therotation speed of the polygon mirror for C color is stabilized. Phasechange is controlled after waiting for stabilizing time Tc1 during whicha rotation of the polygon mirror is stabilized. With regard to the sheetfor second page fed out of tray 2, image forming for C color is startedafter waiting for stabilizing time Tc2 during which a rotation of thepolygon mirror for C color is stabilized.

Further, KTV start timing shown in FIG. 18(G) is for a signal that risesin synchronization with KIDX signal, where an unillustrated KIDX counteris started, and the number of pulses for KIDX signal is counted KVVsignal shown in FIG. 18(H) is a signal that rises in synchronizationwith KIDX signal shown in FIG. 18(I). During the period of “H” level ofthe KVV signal, an image in K color is formed on a sheet coming fromtray 1, and after completion of the foregoing, there is made control forchanging a rotation speed of a polygon mirror for forming an image in Kcolor.

In this case, a frequency of the KIDX signal is fluctuated until therotation speed of the polygon mirror for K color is stabilized. Phasechange is controlled after waiting for stabilizing time Tk1 during whicha rotation of the polygon mirror is stabilized. With regard to the sheetfor second page fed out of tray 2, image forming for K color is startedafter waiting for stabilizing time Tk2 during which a rotation of thepolygon mirror for K color is stabilized. In the example of imageforming operations in the case of switching trays mentioned above,controls of rotation speed of polygon mirror for forming an image ineach of Y, M and C colors and of a phase are practiced after the controlof rotation speed of a polygon mirror for forming an image in K colorhas been completed, because it is carried out based on KIDX signals.

In association with the aforesaid control of a polygon mirror, a laserbeam scanning apparatus is disclosed in Patent Document 1. In this laserbeam scanning apparatus, there is provided a rotation phase calculatingsection that calculates a time difference between an optical beamdetection signal corresponding to a reference polygon mirror and anoptical beam detection signal [corresponding to a polygon mirror otherthan the reference polygon mirror, and compares phase control data basedon the time difference with phase control data corresponding to areference polygon mirror, to generate a rotation frequency. By providingsuch rotation phase calculating section, an orientation of the mirrorsurface of the polygon mirror can be controlled simply.

Patent Document 1: Unexamined Japanese Patent Application PublicationNO. 9-230273 (FIG. 1 on page 5)

Incidentally, in the image forming apparatus applied by the inventors ofthe present invention, there is employed a method to correctmagnifications for the front face and the rear face by changing rotationspeed and phase of the polygon mirror by the use of pseudo indexsignals.

Each of FIGS. 19 (A)-18(O) is a time chart showing an example ofoperations (for Y color) in the case of correcting magnifications forthe front face and the rear face of a color image forming apparatus.

A VTOP signal shown in FIG. 19(A) is a signal that rises insynchronization with YIDX signal shown in FIG. 19(F) after a leadingedge of the sheet fed out of tray 1 is detected. YVV start timing shownin FIG. 19(D) is for a signal that rises in synchronization with YIDXsignal, where an unillustrated YIDX counter is started, and the numberof pulses for YIDX signal is counted. A YVV signal shown in FIG. 19(E)is a signal that rises in synchronization with YIDX signal shown in FIG.19(F). During the period of “H” level of the YVV signal, an image in Ycolor is formed on a sheet coming from tray 1.

The control for changing a rotation speed of the polygon mirror forforming an image in Y color is carried out after completion of Y colorimage forming on the front face of the sheet, namely, after KVV signalshown in FIG. 19(H) has risen. In this case, a frequency of the YIDXsignal is fluctuated until the rotation speed of the polygon mirror forY color is stabilized. Phase change is controlled after waiting forstabilizing time Ty1′ during which a rotation of the polygon mirror isstabilized. With regard to the rear face of the sheet, image forming forY color is started after waiting for stabilizing time Ty2′ during whicha rotation of the polygon mirror for Y color is stabilized.

During the period of “H” level of the MVV signal shown in FIG. 19(H), animage in M color is formed on the front face of the sheet, and aftercompletion of the foregoing, there is practiced a control for changing arotation speed of a polygon mirror for forming an image in M color. Inthis case, a frequency of the MIDX signal is fluctuated until therotation speed of the polygon mirror for M color is stabilized. Phasechange is controlled after waiting for stabilizing time Tm1′ duringwhich a rotation of the polygon mirror is stabilized. With regard to therear face of the sheet, image forming for M color is started afterwaiting for stabilizing time Tm2′ during which a rotation of the polygonmirror for M color is stabilized.

During the period of “H” level of the CVV signal shown in FIG. 19(J), animage in C color is formed on the front face of the sheet, and aftercompletion of the foregoing, there is practiced a control for changing arotation speed of a polygon mirror for forming an image in C color. Inthis case, a frequency of the CIDX signal is fluctuated until therotation speed of the polygon mirror for C color is stabilized. Phasechange is controlled after waiting for stabilizing time Tc1′ duringwhich a rotation of the polygon mirror is stabilized. With regard to therear face of the sheet, image forming for C color is started afterwaiting for stabilizing time Tc2′ during which a rotation of the polygonmirror for C color is stabilized.

KVV start timing shown in FIG. 19(L) is for a signal that rises insynchronization with YIDX signal, where an unillustrated KIDX counter isstarted, and the number of pulses for YIDX signal is counted. KVV signalshown in FIG. 19(L) is a signal that rises in synchronization with KIDXsignal shown in FIG. 19(M). During the period of “H” level of the KVVsignal, an image in K color is formed on a sheet coming from tray 1, andafter completion of the foregoing, there is made control for changing arotation speed of a polygon mirror for forming an image in K color.

In this case, a frequency of the KIDX signal is fluctuated until therotation speed of the polygon mirror for K color is stabilized. Phasechange is controlled after waiting for stabilizing time Tk1′ duringwhich a rotation of the polygon mirror is stabilized. With regard to therear face of th sheet, image forming for K color is started afterwaiting for stabilizing time Tk2′ during which a rotation of the polygonmirror for K color is stabilized. Incidentally, T1 shown in FIG. 19(O)shows a period during which the start timing for each of YVV signal, MVVsignal and CVV signal in the case of image forming on the front face isdetermined with MST-IDX1 serving as a count source, while T2 shows aperiod during which the start timing for each of YVV signal, MVV signaland CVV signal in the case of image forming on the rear face isdetermined with MST-IDX2 serving as a count source. By using pseudoindex signals for correction of magnifications on the front face and therear face as stated above, productivity is improved.

However, a color image forming apparatus relating to the conventionalexample has following problems.

-   (i) Phase changing control cannot be started until the moment when    the polygon mirror arrives at its stable rotation by the instruction    for changing rotation speed of the polygon mirror. Further, even    after practicing the phase changing control, image forming    processing cannot be started until the polygon mirror comes to its    stable rotation. Therefore, when the magnification is corrected, the    switching operation takes time, and productivity for double-face    operations is lowered by conducting correction operation for    magnifications.

In the example of image size correction in the case of tray switchingshown in FIGS. 18(A)-18(I), it is not possible to start succeeding imageformation processing for each color, without waiting for stabilizingtime Ty1 for stabilizing polygon mirror rotation for Y-color, afterY-color image formation processing, without waiting for stabilizing timeTm1+Tm2 for stabilizing polygon mirror rotation for M-color, afterM-color image formation processing, without waiting for stabilizing timeTc1+Tc2 for stabilizing polygon mirror rotation for C-color, afterC-color image formation processing and without waiting for stabilizingtime Tk1+Tk2 for stabilizing polygon mirror rotation for K-color, afterK-color image formation processing. Therefore, high speed imageformation processing is hampered by waiting for these stabilizing timesTy1, Tm1+Tm, Tc1+Tc2 and Tk1+Tk2.

-   (ii) The aforesaid problems are caused equally even in the case of    switching image formation processing between the front face and rear    face by using pseudo index signals shown in FIGS. 19(A)-19(O). In    this case, it is not possible to start succeeding image formation    processing for each color, without waiting for stabilizing time    Ty1′+Ty2′ for stabilizing polygon mirror rotation for Y-color, after    Y-color image formation processing, without waiting for stabilizing    time Tm1′+Tm2′ for stabilizing polygon mirror rotation for M-color,    after M-color image formation processing, without waiting for    stabilizing time Tc1′+Tc2′ for stabilizing polygon mirror rotation    for C-color, after C-color image formation processing and without    waiting for stabilizing time Tk1′+Tk2′ for stabilizing polygon    mirror rotation for K-color, after K-color image formation    processing. Therefore, high speed image formation processing is    hampered by waiting for these stabilizing times Ty1′+Ty2′,    Tm1′+Tm2′, Tc1′+Tc2′ and Tk1′+Tk2′.-   (iii) In the laser beam apparatus seen in Patent Document 1, there    is employed a method to generate polygon clock by comparing a    counter cycle and a count value with a start-up point value    calculated from a phase difference of detector pulse signals (index    signals), concerning phase control of a polygon mirror. Even in this    method, it is not possible to start color image forming processing    for the succeeding page, without waiting stabilizing time after    controlling a phase of a polygon mirror until its rotation is    stabilized. Therefore, productivity in operations for image size    correction is lowered, and continuous high speed processing for    color images is prevented.

With the foregoing as a background, the invention has solved theaforesaid problems, and its objective is to provide an image formingapparatus wherein a decline of productivity in the course of correctingimage size can be controlled, and continuous high speed processing forcolor images can be carried out.

SUMMARY

For solving the problems stated above, the first image forming apparatusreflecting a feature of the present invention is an image formingapparatus for forming color images comprising at least two or morecolors, having a function of magnification correction of image size byone page unit, the image forming apparatus including:

an image carrier;

a polygonal mirror rotator independently provided for each color; and

a controller which simultaneously conducts first control for changingrotation speed of the polygonal mirror rotator in order for changingimage size in a sub-scanning direction perpendicular to a main scanningdirection, and second control for correcting a correction amount forcolor registration error depending on magnification correction of imagesize, and for adjusting a rotating phase of the polygonal mirror rotatordepending on the corrected correction amount for color registrationerror,

where the main scanning direction is a direction in which the imagecarrier is scanned with an exposure beam coming from the polygonalmirror rotator.

In the first image forming apparatus, when forming images by correctingmagnification in terms of image sizes by one page unit, the controllerconducts simultaneously control for changing rotation speed of thepolygonal mirror rotator for changing image size in the sub-scanningdirection and control for correcting a correction amount for colorregistration error depending on correction of magnification for imagesizes, and for adjusting a rotating phase of the polygonal mirrorrotator depending on a correction amount for color registration errorafter the correction.

It is therefore possible to shorten a stabilizing time during which therotation of the polygonal mirror rotator is stabilized, compared with anoccasion wherein speed control and phase control of the polygonal mirrorrotator are carried out in succession.

The second image forming apparatus reflecting another aspect of thepresent invention is an image forming apparatus for continuously formingcolor images comprising at least two or more colors, having a functionof magnification correction of image size by one page unit, the imageforming apparatus including:

a polygonal mirror rotator which is provided independently for eachcolor image forming unit;

an image carrier on which a latent image is formed by an exposure beamscanned by the polygonal mirror rotator and the latent image isdeveloped to be a color image; and

a controller comprising:

-   -   a color registration error detecting section which detects color        registration error on each color image formed on the image        carrier;    -   a color registration error correcting section which corrects the        color registration error depending on an amount of the color        registration error obtained from the color registration error        detecting section; and    -   a calculating section which calculates a rising edge and a        falling edge of a drive clock signal controlling a rotation        speed of the polygonal mirror rotator for a succeeding page,        based on an amount of phase control calculated by correcting an        amount of color registration error correction after correction        by the color registration error correcting section depending on        an amount of magnification correction, on an output value of a        counter provided and independently controlled for each color to        determine a drive clock signal cycle that controls a rotation        speed of the polygonal mirror rotator, on a phase difference        between a first main scanning basis signal generated by        detecting an exposure beam scanned by the polygonal rotator for        a first color image forming unit immediately before conducting        magnification correction for image sizes with a sensor arranged        in a scanning optical path and the second main scanning basis        signal generated by detecting an exposure beam scanned by the        polygonal rotator for a second color image forming unit with a        sensor arranged in a scanning optical path, and on a phase        difference between a first base point of a count cycle of a        counter for generating a drive clock signal of the polygonal        mirror rotator for each of the first and second color image        forming units immediately before conducting magnification        correction for image size and a second base point of a count        cycle after the magnification correction for image size,

wherein the controller executes polygonal mirror rotator drive controlin a case of magnification correction for image sizes by the drive clocksignal, which controls a rotation speed of the polygonal mirror rotator,generated based on an output of the calculating section.

In the second image forming apparatus, when correcting an image size byone page unit, an image is formed on the image carrier by an exposurebeam oscillated by the polygonal mirror rotator, and the image isdeveloped to be a color image. The color registration error detectionsection detects color registration errors of each color image formed onthe image carrier. The color registration error correction sectioncorrects color registration errors depending on an amount of detectionof color registration errors obtained from the color registration errordetection section. On the assumption of the foregoing, the calculatingsection calculates a rising edge and a falling edge of drive clocksignals controlling a rotation speed of the polygonal mirror rotator forthe succeeding page based on an amount of phase control calculated bycorrecting an amount of correction of color registration errorsdepending on an amount of magnification adjustment, an output value of acounter that is provided independently of each color for determining acycle of drive clock signal and is controlled independently, a phasedifference between the first main scanning basis signal immediatelybefore conducting magnification correction for image sizes and thesecond main scanning basis signal, and on a phase difference between abase point of a count cycle of the counter for generating drive clocksignal of the polygonal mirror rotator and a base point of a count cycleunder the condition of count cycle after correction of magnification forimage size, in the controller.

For example, when images are formed in the order of the first, second,third and fourth color image forming units wherein the earliest onecomes first, the controller controls rotational phase for each polygonalmirror rotator so that the second color image forming unit may use abase point of a count cycle of a counter for generating drive clocksignals of the polygonal mirror rotator of the first color image formingunit as a base, the third color image forming unit may use a base pointof a count cycle of a counter for generating drive clock signals of thepolygonal mirror rotator of the second color image forming unit as abase, and the fourth color image forming unit may use a base point of acount cycle of a counter for generating drive clock signals of thepolygonal mirror rotator of the third color image forming unit as abase.

Therefore, compared with a conventional method, it is possible toshorten stabilizing time during which a rotation of the polygonal mirrorrotator is stabilized, because speed control and phase control of thepolygonal mirror rotator can be carried out simultaneously.

The third image forming apparatus reflecting another aspect of thepresent invention is a tandem type color image forming apparatus havinga function to correct image sizes by one page unit and being capable offorming color images composed of at least two or more colorscontinuously, the image forming apparatus including:

a polygonal mirror rotator which is provided independently for eachcolor image forming unit;

an image carrier on which a latent image is formed by an exposure beamscanned by the polygonal mirror rotator and the latent image isdeveloped to be a color image; and

a controller comprising:

-   -   a color registration error detecting section which detects color        registration error on each color image formed on the image        carrier;    -   a color registration error correcting section which corrects the        color registration error depending on an amount of the color        registration error obtained from the color registration error        detecting section; and    -   a calculating section which calculates a rising edge and a        falling edge of a drive clock signal controlling a rotation        speed of the polygonal mirror rotator for a succeeding page,        based on an amount of phase control calculated by correcting an        amount of color registration error correction after correction        by the color registration error correcting section depending on        an amount of magnification correction, on an output value of a        counter provided and independently controlled for each color to        determine a drive clock signal cycle that controls a rotation        speed of the polygonal mirror rotator, on a phase difference        between a first main scanning basis signal generated by        detecting an exposure beam scanned by the polygonal rotator for        a first color image forming unit immediately before conducting        magnification correction for image sizes with a sensor arranged        in a scanning optical path and the second main scanning basis        signal generated by detecting an exposure beam scanned by the        polygonal rotator for a second color image forming unit with a        sensor arranged in a scanning optical path, and on a phase        difference between a base point of a count cycle for generating        a pseudo index signal, which is obtained by dividing a source        oscillation signal of an original oscillator used in common with        generation of drive clock signal of the polygonal mirror rotator        in practicing rotational phase control of the polygonal mirror        rotator so that the pseudo index signal agrees with one plane        cycle of the polygonal mirror rotator, and a base point of a        counter cycle for generating drive clock signal of the polygonal        mirror rotator for each color unit,

wherein the controller executes polygonal mirror rotator drive controlin a case of magnification correction for image sizes by the drive clocksignal, which controls a rotation speed of the polygonal mirror rotator,generated based on an output of the calculating section.

In the third image forming apparatus, when correcting an image size byone page unit, an image is formed on the image carrier by an exposurebeam oscillated by the polygonal mirror rotator, and the image isdeveloped to be a color image. The color registration error detectionsection detects color registration errors of each color image formed onthe image carrier. The color registration error correction sectioncorrects color registration errors depending on an amount of detectionof color registration errors obtained from the color registration errordetection section. On the assumption of the foregoing, the calculatingsection calculates a rising edge and a falling edge of drive clocksignals controlling a rotation speed of the polygonal mirror rotator forthe succeeding page based on an amount of phase control calculated bycorrecting an amount of correction of color registration errorsdepending on an amount of magnification adjustment, an output value of acounter that is provided independently of each color for determining acycle of drive clock signal and is controlled independently, a phasedifference between the first main scanning basis signal immediatelybefore conducting magnification correction for image sizes and thesecond main scanning basis signal, and on a phase difference between abase point of a count cycle generating pseudo index signal and a basepoint of a counter cycle for generating drive clock signal for thepolygonal mirror rotator of each color unit, in the controller.

Therefore, compared with a conventional method, it is possible toshorten stabilizing time during which a rotation of the polygonal mirrorrotator is stabilized, because speed control and phase control of thepolygonal mirror rotator can be carried out simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, advantages and features of the invention willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings in which:

FIG. 1 is a conceptual diagram showing an example of configuration ofcolor copier 100 as the first embodiment of the invention;

FIG. 2 is a block diagram showing an example of configuration of acontrol system of the color copier 100;

FIG. 3 is a block diagram showing an example of configuration of imagewriting unit 3Y for Y-color image forming shown in FIG. 2 and itsperipheral circuit;

FIG. 4 is a block diagram showing an example of configuration of apolygon mirror drive system for each color image forming;

Each of FIGS. 5 (A)-5 (F) is a time chart showing an example ofoperations (YP-CLK basis time) before magnification correction controlin image forming section 60;

Each of FIGS. 6 (A)-6 (F) is a time chart showing an example ofoperations (YP-CLK signal basis time) after magnification correctioncontrol in image forming section 60;

Each of FIGS. 7(A)-7(I) is a time chart showing an example of operations(Y-color basis) after magnification correction control of color copier100;

FIG. 8 is a block diagram showing an example of configuration of colorcopier 200 as the second embodiment;

FIG. 9 is a block diagram showing an example of configuration of imagewriting unit 3Y′ for Y-color image forming shown in FIG. 8 and itsperipheral circuit;

FIG. 10 is a block diagram showing an example of configuration of apolygon mirror drive system including a pseudo IDX generating circuit;

Each of FIGS. 11(A)-11(E) is a time chart showing an example ofoperations (MST-IDEX basis time) before magnification correction controlin image forming section 60′;

Each of FIGS. 12(A)-12(E) is a time chart showing an example ofoperations (MST-IDEX basis time) after magnification correction controlin image forming section 60′;

Each of FIGS. 13(A)-13(M) is a time chart showing an example ofoperations (MST-IDEX signal basis) after magnification correctioncontrol of color copier 200;

FIG. 14 is a block diagram showing an example of configuration of acontrol system in color copier 300 relating to the third embodiment;

FIG. 15 is a block diagram showing an example of configuration of imagewriting unit 3Y″ for Y-color image forming extracted from FIG. 14 andits peripheral circuit;

Each of FIGS. 16(A)-16(O) is a time chart showing an example ofoperations after magnification correction control of color copier 300;

Each of FIGS. 17(A) and 17(B) is a diagram illustrating an example ofshrinkage of a sheet size in the case of double-face image forming;

Each of FIGS. 18(A)-18(I) is a time chart showing an example of imagesize correction (K-color basis) in the case of switching trays in animage writing unit for each of Y-color, M-color, C-color and K-color inconventional examples; and

Each of FIGS. 19(A)-19(O) is a time chart showing an example ofoperations (Y-color basis) in the case of front-face/rear-facemagnification correction of a color image forming apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An image forming apparatus relating to an example of the invention willbe explained as follows, referring to the drawings.

Embodiment 1

FIG. 1 is a conceptual diagram showing an example of configuration of asection of color copier 100 as the first embodiment of the invention.

Color copier 100 shown in FIG. 1 is an example of the first, second orthird image forming apparatus representing an apparatus that has afunction to correct image sizes by one page unit and is capable offorming continuously color images composed of at least two or morecolors. An image forming apparatus relating to the invention may also beapplied to a color printer, a facsimile machine and theirmultifunctional machine, in addition to the color copier 100.

The color copier 100 is composed of copier main body 101 and imagereading unit 102. The image reading unit 102 composed of automaticdocument feeder 201 and document image scanning exposure unit 202 isarranged on the top of the copier main body 101. Document 30 placed on adocument table of the automatic document feeder 201 is conveyed by anunillustrated conveyer, and thereby images on one side or two sides ofthe document are subjected to scanning exposure by the optical system ofthe document image scanning exposure unit 202, and an incident lightreflecting document images is read by line image sensor CCD.

Analog image signals converted photoelectrically by line image sensorCCD are subjected to analog processing, A/D conversion, shadingcorrection and image compression processing in an unillustrated imageprocessing section, to become digital image data Din. The image data Dinare sent to image writing units (laser writing units) 3Y, 3M, 3C and 3Kconstituting image forming section 60, after being converted to imagedata Dy, Dm, Dc and Dk for image forming for Y-color, M-color, C-colorand K-color.

The aforesaid automatic document feeder 201 reads contents in document30 fed from the document table by one effort continuously, andaccumulates contents of the document in a memory section (electronic RDHfunction). This electronic RDH function is used conveniently whencopying contents of many documents by a copying function, or whensending many documents 30 by a facsimile function.

The copier main body 101 constitutes a tandem type color image formingapparatus, and is provided with four image forming units (image formingsystems) 10Y, 10M, 10C and 10K, endless intermediate transfer belt 6, asheet conveying section including a sheet re-feeding mechanism (ADUmechanism), fixing unit 17 for fixing a toner image and with sheetfeeding section 20 that feeds a transfer material (hereinafter referredto as a sheet) to an image forming system. The sheet feeding section 20is provided below the image forming system. The sheet feeding section 20is composed, for example, of three sheet feeding trays 20A, 20B and 20C.Sheet P fed out of the sheet feeding section 20 us conveyed to the lowerpart of the image forming unit 10K.

Image forming units 10Y, 10M, 10C and 10K constitute image formingsection 60, and a polygon mirror and a photoreceptor drum are providedfor each color, and they form color images on prescribed sheet P basedon main scanning basis signal (hereinafter referred to as index signal)and/or on pseudo main scanning basis signal).

For example, image forming unit 10Y has polygon mirror 42Y andphotoreceptor drum (image carrier) 1Y, image forming unit 10M haspolygon mirror 42M and photoreceptor drum (image carrier) 1M, imageforming unit 10C has polygon mirror 42C and photoreceptor drum (imagecarrier) 1C and image forming unit 10K has polygon mirror 42K andphotoreceptor drum (image carrier) 1K. Each of the polygon mirrors42Y-42K is provided independently of others, and scanning beams of thepolygon mirrors 42Y-42K form latent images which are developed throughdevelopment into color images.

In this example, the image forming unit 10Y for forming a yellow (Y)color image has therein photoreceptor drum 1Y for forming a Y-colortoner image, charging unit 2Y for Y-color image forming arranged aroundthe photoreceptor drum 1Y, image writing unit 3Y, developing unit 4Y andcleaning section 8Y for the image carrier.

The image forming unit 10M for forming a magenta (M) color image hastherein photoreceptor drum 1M for forming a M-color toner image,charging unit 2M for M-color image forming, image writing unit 3M,developing unit 4M and cleaning section 8M for the image carrier. Theimage forming unit 10C for forming a cyan (C) color image has thereinphotoreceptor drum 1C for forming a C-color toner image, charging unit2C for C-color image forming, image writing unit 3C, developing unit 4Cand cleaning section 8C for the image carrier. The image forming unit10K for forming a black (K) color image has therein photoreceptor drum1K for forming a K-color toner image, charging unit 2K for K-color imageforming, image writing unit 3K, developing unit 4K and cleaning section8K for the image carrier.

A latent image forming section is constituted by a combination ofcharging unit 2Y and image writing unit 3Y, a combination of chargingunit 2M and image writing unit 3M, a combination of charging unit 2C andimage writing unit 3C, and a combination of charging unit 2K and imagewriting unit 3K. Development by each of developing units 4Y, 4M, 4C and4K is carried out by reversal development in which developing bias wherealternating voltage is superimposed on direct voltage whose polarity isthe same as that of working toner (negative polarity in the presentexample) is impressed. The intermediate transfer belt 6 is trained aboutplural rollers, to be supported rotatably, and a Y-color toner image, aM-color toner image, a C-color toner image, and a K-color toner imageformed respectively on respective photoreceptor drums 1Y, 1M, 1C and 1Kare transferred onto the intermediate transfer belt 6.

An outline of image forming process will now be explained as follows.Images each having a different color formed respectively by imageforming units 10Y, 10M, 10C and 10K are transferred onto rotatingintermediate transfer belt 6(primary transfer) in order by primarytransfer rollers 7Y, 7M, 7C and 7K on each of which primary transferbias (not shown) having polarity opposite to that of working toner(positive polarity in the present example), thus, color toner images aresuperimposed to form a color image. The color image is transferred ontosheet P from the intermediate transfer belt 6.

Sheet P loaded in each of sheet feeding trays 20A, 20B and 20C is fed byfeed out roller 21 and sheet feeding roller 22A which are provided oneach of the sheet feeding trays 20A, 20B and 20C to be conveyed tosecondary transfer roller 7A through conveyance rollers 22B, 22C, 22Dand registration rollers 23 and 28, whereby, color images aretransferred collectively onto the surface on one side (front face) ofsheet P (secondary transfer).

The sheet P onto which the color image has been transferred is subjectedto fixing processing by fixing unit 17, and is interposed by sheetejection rollers 24 to be conveyed to sheet ejection tray 25. Tonerremaining on a circumferential surface on each of photoreceptor drums1Y, 1M, 1C and 1K after transferring is removed by each of image carriercleaning sections 8Y, 8M, 8C and 8K, to be ready for the succeedingimage forming cycle.

In the case of double-face image forming, sheet P which has beensubjected to image forming on the surface (front face) of its one sideand has been ejected from fixing unit 17 is branched from a sheetejecting path by branch section 26, then, passes through lowercirculating sheet path 27A to be reversed inside out by reversingconveyance path 27B representing a sheet re-feeding mechanism (ADUmechanism), and passes through sheet re-feeding conveyance section 27Cto join at sheet feeding roller 22D. The sheet P which has been reversedand conveyed passes through registration rollers 23 and 28 to beconveyed again to secondary transfer roller 7A where color images (colortoner images) are transferred collectively onto the surface (rear face)on the other side of the sheet P.

In the case of image forming stated above, sheet P to be used includes athin sheet of about 52.3-63.9 kg/m² (1000 sheets), a regular sheet ofabout 64.0-81.4 kg/m² (1000 sheets), a thick sheet of about 83.0-130.0kg/M² (1000 sheets) and a super-thick sheet of about 150.0 kg/m² (1000sheets).

The sheet P onto which the color image has been transferred is subjectedto fixing processing by fixing unit 17, and is interposed by sheetejection rollers 24 to be conveyed to sheet ejection tray 25. On theother hand, after a color image is transferred onto sheet P by secondarytransfer roller 7A, intermediate transfer belt 6 from which theaforesaid sheet P is separated through curvature is cleaned by cleaningsection 8A for an intermediate transfer belt so that residual toner isremoved. In this example, registration sensor 5 representing an exampleof a detection section for color registration error is arranged at theupstream side of the cleaning section 8A, to detect color registrationerrors of each color image formed on the intermediate transfer belt 6.

Copier main body 100 is equipped with controller 15 that conductssimultaneously controlling to change a rotation speed of polygon mirror42Y or the like for changing an image size in the sub-scanning directionand controlling to adjust rotation phase of the polygon mirror 42Ydepending on an amount of correction for color registration errors afterthe correction (first image forming apparatus). The controller 15constitutes a part of a color registration error correcting section, andit corrects color registration errors depending on an amount ofdetection for color registration error obtained from registration sensor5. For example, a minute amount of registration error in thesub-scanning direction can be adjusted by conducting phase control(which is also called plane phase control) for each of polygon mirrors42Y-42K by the controller 15.

FIG. 2 is a block diagram showing an example of configuration of acontrol system of the color copier 100. The color copier 100 shown inFIG. 2 has therein controller 15 that determines timing to start imageforming on a prescribed surface of sheet P based on basis signal(hereafter, index signal for each color image forming is called YIDX,MIDX, CIDX or KIDX) for forming Y-color image, M-color image, C-colorimage or K-color image. The basis signal in this case means mainscanning basis signal (INDEX signal) to be generated by detecting alaser (exposure) beam oscillated by polygon mirror 42Y of each colorimage forming unit.

Crystal oscillator (source oscillator) 11, image memory 13, imageprocessing section 16, communication section 19, sheet feeding section20, operation panel 48, image forming section 60 and image reading unit102 are connected to the controller 15.

The crystal oscillator 11 oscillates basis clock signal (hereinafterreferred to as CLK1 signal) representing a basis signal in the case ofcolor image forming. The CLK1 signals oscillated by the crystaloscillator 11 are outputted, for example, to image writing units 3Y, 3M,3C and 3K which are respectively for Y-color image forming, M-colorimage forming, C-color image forming and K-color image forming.

The controller 15 has therein ROM (Read Only Memory) 53, RAM (RandomAccess Memory) 54 for work and CPU (Central Processing Unit; centralprocessing unit) 55. System program data for controlling the overallcopier and information for controlling a rotation speed and a phase ofpolygon mirror 42 are stored in the ROM 53. These pieces of informationinclude counter control signals (hereinafter referred to as CNTPRDsignals) and phase control signals (hereinafter referred to as PHASEsignals). The RAM 54 stores temporarily control command inimplementation of various modes.

When power supply is turned on, CPU 55 starts the system by readingsystem program data from ROM 53, and controls the overall copier. TheCPU55, for example, executes control of color image forming on aprescribed surface of sheet P based on CLK1 signal and YIDX signal, whenforming a color image on prescribed sheet P on a basis of Y-color. Withregard to the YIDX signal, its cycle varies depending on rotation speedcontrol and phase control for polygon mirror 42Y. The CPU 55 determinesimage leading edge signal in color image forming processing from a frontface to a rear face of sheet P (hereinafter referred to as VTOP signal)and VTOP signal in color image forming processing in the case ofswitching from tray 1 to tray 2. The VTOP signal is a signal forsynchronizing timing for conveying sheet P with timing for imageforming.

The image forming section 60 is equipped with image writing units 3Y,3M, 3C and 3K which are respectively for image forming for Y-color,M-color, C-color and K-color. In the color image forming, the CPU 55establishes frequency control signal Sg, CNTPRD signal and PHASE signalon each of the image writing units 3Y, 3M, 3C and 3K. In Y-image writingunit 3Y, image data Dy for Y-color image forming are inputted from imagememory for Y-color image forming, and actions are taken to form Y-colortoner images based on frequency control signal Sg, CNTPRD signal, PHASEsignal, CLK1 signal and an unillustrated YIDX signal. The YIDX signal isa basis signal in the case of controlling a rotation speed and a phaseof polygon mirror 42Y for Y-color image forming and thereby scanningphotoreceptor drum 1Y with a laser beam, and it is a signal obtained bydetecting a laser beam reflected on polygon mirror 42Y.

Equally, in M-image writing unit 3M, image data Dm for M-color imageforming are inputted from image memory for M-color image forming, andactions are taken to form M-color toner images based on frequencycontrol signal Sg, CNTPRD signal, PHASE signal, CLK1 signal and MIDXsignal. The MIDX signal is a basis signal in the case of controlling arotation speed and a phase of polygon mirror 42M for M-color imageforming and thereby scanning photoreceptor drum 1M with a laser beam,and it is a signal obtained by detecting a laser beam reflected onpolygon mirror 42M.

In C-image writing unit 3C, image data Dc for C-color image forming areinputted from image memory for C-color image forming, and actions aretaken to form C-color toner images based on frequency control signal Sg,CNTPRD signal, PHASE signal, CLK1 signal and CIDX signal. The CIDXsignal is a basis signal in the case of controlling a rotation speed anda phase of polygon mirror 42C for C-color image forming and therebyscanning photoreceptor drum 1C with a laser beam, and it is a signalobtained by detecting a laser beam reflected on polygon mirror 42C.

In K-image writing unit 3K, image data Dk for K-color image forming areinputted from image memory for K-color image forming, and actions aretaken to form K-color toner images based on frequency control signal Sg,CNTPRD signal, PHASE signal, CLK1 signal and KIDX signal. The KIDXsignal is a basis signal in the case of controlling a rotation speed anda phase of polygon mirror 42K for K-color image forming and therebyscanning photoreceptor drum 1K with a laser beam, and it is a signalobtained by detecting a laser beam reflected on polygon mirror 42K.

In this example, the controller 15 executes color image forming controlon a prescribed surface of sheet P based on YIDX signal and VTOP signal.Due to this, it is possible to correct image size on each of the frontface and the rear face of sheet P, even when sheet P shrinks after imageforming on the front face, when forming images on both the front faceand the rear face of the sheet. It is further possible to correct animage size on a different sheet, even when a type of a sheet on tray 1is different from that of a sheet on tray 2, when forming color imagesafter switching sheet feeding from tray 1 to tray 2.

Incidentally, operation panel 48 is connected to the controller 15, andhas therein operation section 14 composed of a touch panel and displaysection 18 composed of liquid crystal display panel, both of which arenot illustrated. An input section of a type of GUI (Graphic UserInterface) is used for the operation panel 48. A power supply switch isprovided on the operation panel 48. The display section 18 conductsdisplay operations, interlocking with, for example, operation section14.

The operation panel 48 is operated when selecting image formingconditions and selecting sheet feeding trays 20A-20C. For example,operation section 14 is operated when selecting a type of sheet P (sheettype) from a regular sheet, a recycled sheet, coated paper and OHT sheetand when selecting a sheet feeding tray storing therein the selectedsheet from sheet feeding trays 20A-20C, thus, image forming conditionsare established. Incidentally, the image forming conditions establishedby the operation panel 48 and information of the selected sheet feedingtray are outputted to the CPU 55 as operation data D3.

The aforesaid controller 15 executes color image forming on a prescribedsurface of sheet P based on operation data D3 outputted from operationsection 14 or on information received through communication section 19.For example, the aforesaid controller 15 executes processing to adjustan image size between the front face and the rear face of sheet P andprocessing to adjust a position between the front face and the rear faceof sheet P, corresponding to a type of set sheet P or to set sheetfeeding trays 20A-20C.

Image reading unit 102 is connected to the controller 15, and it readsimages from document 30 shown in FIG. 1 to output image data Din (eachcolor component data for R, G and B) for digital color to the controller15. In the controller 15, image data Din are stored in image memory 13.Image processing section 16 reads image data Din from image memory 13,and conducts processing to convert color component data for R, G and Binto image data Dy for Y-color image forming, image data Dm for M-colorimage forming image data Dc for C-color and image data Dk for K-colorimage forming. Image data Dy, Dm, Dc and Dk respectively for Y-colorimage forming, M-color image forming, C-color image forming and K-colorimage forming are stored in image memory 13 or in an unillustrated imagememory for Y-color image forming, M-color image forming, C-color imageforming and K-color image forming.

Communication section 19 is connected to a communication line such asLAN, and is used when communicating with outside computers. When thecolor copier 100 is used as a printer, the communication section 19 isused to receive print data Din′ from outside computers, in the mode ofprinting operation. Incidentally, print data Din′ include image formingconditions and information of selecting sheet feeding trays. Thosereceived from outside computers through communication section 19 mayalso be used as the image data Dy, Dm, Dc and Dk respectively forY-color, M-color, C-color and K-color image forming.

Sheet feeding section 20 is connected to an unillustrated motor fordriving sheet feeding trays 20A-20C, and it controls rotation of themotor based on sheet feeding control signal Sf, and operates to conveysheet P fed out of the sheet feeding tray 20A, 20B or 20C to the imageforming system. The sheet feeding control signal Sf is supplied to sheetfeeding section 20 from the controller 15.

FIG. 3 is a block diagram showing an example of configuration of imagewriting unit 3Y for Y-color image forming shown in FIG. 2 and itsperipheral circuit.

Y-color image writing unit 3Y shown in FIG. 3 is connected to crystaloscillator 11 and to CPU 55. The Y-color image writing unit 3Y iscomposed, for example, of crystal oscillator 31, pixel CLK generatingcircuit 32, horizontal synchronizing circuit 33, PWM signal generatingcircuit 34, laser (LD) drive circuit 35, polygon motor 36Y, motor drivecircuit 37Y, index sensor 38Y, polygon drive CLK generating circuit 39Y,timing generator 40, Y-VV (Valid) generating circuit 41 and countercircuit 43Y.

Counter circuit 43Y is one for determining a cycle of YP-CLK signal thatcontrols a rotation speed of polygon mirror 42Y, and it counts thenumber of pulses of CLK signals based on Y-CNTPRD signal, and outputsY-CNT signal of the first cycle and Y-ORG signal of the second cycle.CLK1 signal is outputted to counter circuit 43Y from crystal oscillator11. The Y-CNTPRD signal is a signal to establish a target count value ofcounter circuit 43Y, and it is a signal to establish a cycle of YP-CLKsignal, namely, a speed of polygon motor 36Y. Y-CNTPRD signal isoutputted to counter circuit 43Y from CPU 55 in the case of imageforming on the front face and the rear face. This signal is used forcontrolling a rotation speed of polygon mirror 42Y. Y-CNT signal andY-ORG signal are outputted to polygon drive CLK generating circuit 39Yfrom counter circuit 43Y.

To counter circuit 43Y and CPU 55, there is connected polygon drive CLKgenerating circuit 39Y, and Y-PHASE signal, Y-CNT signal, Y-ORG signaland YIDX signal are inputted to be processed to generate polygon driveclock signal (YP-CLK signal) for Y-color image forming. The Y-PHASEsignal is a signal that establishes an amount of phase adjustment on thepolygon drive CLK generating circuit 39Y, and it is used for controllinga phase of polygon mirror 42Y. Further, on the polygon drive CLKgenerating circuit 39Y, rotation speeds of polygon mirrors 42Y-42K arechanged.

The YIDX signal is outputted to the polygon drive CLK generating circuit39Y from index sensor 38Y. CLK1 signal is outputted to the polygon driveCLK generating circuit 39Y from crystal oscillator 11. An example ofinternal configurations of the polygon drive CLK generating circuit 39Ywill be explained, referring to FIG. 4.

Motor drive circuit 37Y is connected to the polygon drive CLK generatingcircuit 39Y. The motor drive circuit 37Y is connected to polygon motor36Y, to drive the polygon motor 36Y based on YP-CLK signal. Polygonmirror 42Y is mounted on the polygon motor 36Y to be rotated by drivepower of the polygon motor 36Y in the main scanning direction.

Laser beam LY radiated from an unillustrated diode is oscillated formain scanning when polygon mirror 42Y is rotated for photoreceptor drum1Y rotating in the sub-scanning direction, in the aforesaid LD drivecircuit 36, whereby an electrostatic latent image is formed on thephotoreceptor drum 1Y. The electrostatic latent image formed on thephotoreceptor drum 1Y is developed with toner member for Y-color imageforming. A Y-color toner image on the photoreceptor drum 1Y istransferred onto intermediate transfer belt 6 rotating in thesub-scanning direction (primary transfer).

In the mean time, crystal oscillator 31 oscillates basis clock signals(hereinafter referred to as CLK2 signals) and outputs them to pixel CLKgenerating circuit 32 which is connected to the crystal oscillator 31.The pixel CLK generating circuit 32 constitutes a pixel clock frequencychanging section, and operates to generate pixel clock signals forY-color image forming (hereinafter referred to as G-CLK signals) basedon frequency control signal Sg outputted by CPU 55 and thereby to outputto horizontal synchronizing circuit 33.

The pixel CLK generating circuit 32 changes a pixel clock frequencydepending on an amount of change in rotation speed for each of polygonmirrors 42Y-42K and an amount of adjustment of lateral magnification.For example, a value obtained by multiplying frequency f0 of G-CLKsignal in the case of image forming on the front face and (L/L′) (W/W′)together is established as Y-color image forming pixel CLK frequency fin the case of image forming on the rear face. The aforesaid pixel CLKgenerating circuit 32 and the polygon drive CLK generating circuit 39Yconstitute a magnification correcting section which correctsmagnifications in terms of image sizes by one page unit.

The horizontal synchronizing circuit 33 is connected to the pixel CLKgenerating circuit 32 and to PMW signal generating circuit 34, anddetects horizontal synchronizing signal Sh based on YIDX signal tooutput to the PMW signal generating circuit 34. The YIDX signal isoutputted from index sensor 38Y for Y-color image forming not only tothe horizontal synchronizing circuit 33 but also to polygon drive CLKgenerating circuit 39Y. The index sensor 38Y is composed of alight-receiving element.

The PMW signal generating circuit 34 inputs image data Dy for Y-colorimage forming from image memory 83 for Y-color image forming, andmodulates the image data Dy in terms of pulse width to output laserdrive signal Sy for Y-color image forming to LD drive circuit 35. Theaforesaid PMW signal generating circuit 34 is connected with LD drivecircuit 35. The LD drive circuit 35 is connected with an unillustratedlaser diode. The LD drive circuit 35 drives the laser diode based onlaser drive signal Sy, and generates laser beam LY for Y-color imageforming to radiate to polygon mirror 42Y.

To the aforesaid crystal oscillator 11, connected is timing signalgenerator 40 for determining image forming start timing for Y-colorimage forming. The timing signal generator 40 is further connected withCPU 55, and counts the number of pulses of YIDX signals based on VTOPsignals outputted from the CPU 55 in the case of image forming on thefront face, for example, to determine image forming start timing forY-color image forming on the front face of the sheet based on the numberof the counted pulses. Concurrently with this determining of imageforming start timing for Y-color image forming, image forming startsignals (hereinafter referred to as STT signals) are outputted to Y-VVcreating circuit 41Y.

The Y-VV creating circuit 41Y counts the number of pulses of YIDXsignals based on STT signals outputted from the timing signal generator40 to create sub-scanning effective area signal (hereinafter referred toas YTV signal) for Y-color image forming on the front face of the sheetbased on the number of the counted pulses. The YVV signal is outputtedto image memory 83 for Y-color image forming.

To the aforesaid PMW signal generating circuit 34, there is connectedimage memory 83 for Y-color image forming, so that image data Dy forY-color image forming may be read out based on YVV signal in the case offorming images on both the front face and the rear face of the sheet.With regard to the image data Dy, image data for R, G and B colors areread out from image memory 13 shown in FIG. 2 in image processingsection 16, and the image data for R, G and B colors represent one ofimage data for Y, M, C and K colors converted in terms of a color.

Further, the timing signal generator 40 counts the number of pulses ofYIDX signals based on VTOP signals outputted from CPU 55, immediatelybefore the start of image forming on the rear face f the sheet, forexample, to determine image forming start timing for Y-color imageforming on the rear face of the sheet based on the number of the countedpulses. Concurrently with this determining of image forming start timingfor Y-color image forming, STT signals (image forming start signals) areoutputted to Y-VV creating circuit 41Y.

Y-VV creating circuit 41Y counts the number of pulses of YIDX signalsbased on STT signals outputted from timing signal generator 40, tocreate YVV signals for Y-color image forming on the rear face of thesheet based on the number of the counted pulses. YVV signals areoutputted to image memory 83 for Y-color image forming.

In the mean time, since each of other image writing units 3M, 3C and 3Khas also the configuration and function which are the same as those inthe foregoing, descriptions for them will be omitted. In the presentexample, an explanation has been given by including crystal oscillator31, pixel CLK generating circuit 32, horizontal synchronizing circuit33, PWM signal generating circuit 34, polygon drive CLK generatingcircuit 39Y, timing generator 40, Y-VV generating circuit 41 and counter43Y in the image writing unit 3Y. However, the invention is not limitedto this, and these circuit elements may also be included in imageprocessing section 16 or in controller 15 for the configuration.

In that case, it is also possible to employ a configuration wherein CPU55 is caused to have functions of the timing generator 40, VTOP signalis started based on CLK1 signal in the case of image forming on thefront face of the sheet, the number of pulses of YIDX signals is countedbased on the VTOP signal, and first image forming start timing forY-color on the front face of the sheet is determined based on the numberof counted pulses. Based on the STT signal (image forming start signal)determined here, the number of pulses of YIDX signals for Y-color imageforming is counted, and image writing unit 3Y is controlled so that YVVsignal for Y-color image forming on the front face of the sheet may becreated based on the number of counted pules.

In the case of image forming on the rear face of the sheet, CPU 55starts VTOP signal based on CLK1 signal, then, counts the number ofpulses of YIDX signals based on the VTOP signal, and determines firstimage forming start timing for Y-color for the rear face of the sheetbased on the number of counted pulses.

The CPU 55 may also be made up to control input and output of imagewriting unit 3Y so that the number of pulses of YIDX signal for eachcolor image forming are counted based on the determined image formingstart timing, and YVV signal for Y-color image forming on the rear faceof the sheet is created based on the number of the counted pulses.

In the present example, the CPU 55 controls a frequency of YP-CLK signalfor each color in the order wherein image forming on the front face ofthe sheet for each of respective colors is completed, to establish arotation speed of polygon mirror 42Y for the rear face of the sheet, andthen, executes phase control for the MST-IDX.

If the control is operated as stated above, it is possible to carry outthe control such as the rotation speed change and phase change of thepolygon mirror 42Y, after completion of image forming for respectivecolors, based on MST-IDX signal established to a prescribed cycle,without depending on IDX signal of basis color.

Due to this, it is possible to carry out the control such as therotation speed change and phase change of the polygon mirror for thatcolor image forming, by waiting neither stabilization of the rotationspeed of polygon mirror 42K established to the basis color, noradjustment of timing until the start of image forming for all of othercolors.

FIG. 4 is a block diagram showing an example of configuration of apolygon mirror drive system for each color image forming, and it is adiagram wherein polygon mirror drive system (hereinafter referred tosimply as Y, M, C or K unit) for each color image forming is extractedfrom image writing units 3Y, 3M, 3C and 3K for respective colors shownin FIG. 2.

Y unit 3Y shown in FIG. 4 is composed of counter circuit 43Y, polygondrive CLK generating circuit 39Y, motor drive circuit 37Y, polygon motor36Y and index sensor 38Y.

The counter circuit 43Y determines output timing of YP-CLK signal fordriving polygon motor 36Y (polygon mirror 42Y). This output timing meansa rising edge and falling edge of YP-CLK signal. When Y-color imageforming is made to be a basis, CPU 55 determines output timing of YP-CLKsignal for the succeeding page, based on output value Y-CNT signal ofcounter circuit 43Y, a phase difference between YIDX signal and YIDXsignal, a phase difference between a base point of count cycle of Y-ORGsignal by counter circuit 43Y and a base point of count cycle of Y-ORGsignal by counter circuit 43Y, and on Y-PHASE signal showing an amountof phase control of polygon mirror 42Y. The amount of phase controlmentioned here means one which is calculated by correcting an amount ofcorrection for color registration errors before correction ofmagnification for image size in accordance with an amount of adjustmentof magnification.

The CPU 55 controls individually a count cycle established independentlyof each of polygon mirrors 42Y-42K, based on CLK1 signal though countercircuits 43Y, $#M, 43C and 43K. The CPU 55 controls counter circuits43Y, 43M, 43C and 43K, based on YP-CLK signal outputted from polygondrive CLK generating circuit 39Y, so that the count cycle may be thesame regarding image forming of the same page, when driving polygonmotor 36Y, and establishes count cycle individually on each of imageforming units 10Y, 10M, 10C and 10K for each color, to execute speedcontrol.

Polygon drive CLK generating circuit 39Y is connected with countercircuit 43Y, and generates YP-CLK, referring to output values of thecounter circuit 43Y. The polygon drive CLK generating circuit 39Y hastherein phase detection circuit 301 for index use, phase detectioncircuit 302 for counter use and calculating & comparing section 303.

In the phase detection circuit 301 for index, phase difference PYbetween YIDX signal for Y-color image forming and YIDX signal isdetected. In the phase detection circuit 302 for counter, phasedifference AY between a base point of a count cycle of Y-ORG signal bycounter circuit 43Y for Y-color image forming and a base point of acount cycle of Y-ORG signal is detected. To the phase detection circuits301 and 302, there is connected calculating & comparing section 303constituting an example of a calculation section which carry out anoperation for phase difference PY, phase difference AY and Y-PHASE tocalculate an amount of phase adjustment. In the present example, “anamount of phase adjustment=0” is outputted because of Y-color imageforming basis. The calculating & comparing section 303 executes polygonmirror drive control in the case of magnification correction in terms ofan image size, with YP-CLK signal for controlling a rotation speed ofpolygon mirror 42Y generated based on the result of the operation.

To the counter circuit 43M, there is connected polygon drive CLKgenerating circuit 39M which generates MP-CLK, referring to the outputvalue of the counter circuit 43M. The polygon drive CLK generatingcircuit 39M has therein phase detection circuit 304 for index, phasedetection circuit 305 for counter and calculating & comparing section306.

In the phase detection circuit 304 for index, phase difference PMbetween YIDX signal for Y-color image forming and MIDX signal isdetected. The phase difference PM between YIDX signal and MIDX signal inthis case means a phase difference between a main scanning basis signalfor Y-color image writing unit 3Y immediately before conductingmagnification correction in terms of an image size and main scanningbasis signal for M-color image writing unit 3M. In the phase detectioncircuit 305 for counter, phase differences AM and AM′ each between abase point of a count cycle of Y-ORG signal by counter circuit 43Y forY-color image forming and a base point of a count cycle of M-ORG signalfor M-color image forming is detected. This phase difference AM is onebetween a base point of a count cycle of counter circuit 43Y for Y-colorimage writing unit 3Y immediately before conducting magnificationcorrection in terms of an image size and a base point of a count cycleof counter circuit 43M for M-color image writing unit 3M, while, phasedifference AM′ is a phase difference from a base point of a count cyclein the state where counter circuits 43Y and 43M after magnificationcorrection in terms of an image size arrive at a count cycle.

To the phase detection circuits 304 and 305, there is connectedcalculating & comparing section 306 which carries out an operation forphase difference PM, phase differences AM and AM′ as well as M-PHASE tocalculate an amount of phase adjustment. In addition to Y unit 3Y and Munit 3M, C-K units have the same configuration and are equipped with thesame functions. Therefore, explanation for them will be omitted here.

Each of FIGS. 5(A)-5(F) is a time chart showing an example of operations(YP-CLK basis time) before magnification correction control in imageforming section 60. In the present example, there is shown the statebefore magnification correction control in the occasion where YP-CLKsignal is a basis (CNTPRD Y=CNTPRD M=N1).

YIDX signals shown in FIG. 5(A) are outputted from index sensor 38Yshown in FIG. 4 before magnification correction control to phasedetection circuits 301 and 304. Y-CNT signal shown in FIG. 5(B) isoutputted from counter circuit 43Y shown in FIG. 4 to calculating &comparing section 303. In FIG. 5(B), a counter cycle is set by Y-CNTPRDsignal to output value N1.

YP-CLK signal shown in FIG. 5(C) is outputted from calculating &comparing section 303 shown in FIG. 4 to motor drive circuit 37Y. InFIG. 5(C), period (t5-t1) is a clock cycle of YP-CLK signal beforemagnification correction control. At the point in time when countercircuit 43Y counts N1/2, YP-CLK signal is reversed from a high level toa low level.

MIDX signal shown in FIG. 5(D) is outputted from index sensor 38M shownin FIG. 4 before magnification correction control to phase detectioncircuit 304. M-CNT signal shown in FIG. 5. (E) is outputted from countercircuit 43M shown in FIG. 4 to calculating & comparing section 306. InFIG. 5 (E), a counter cycle is set by M-CNTPRD signal to output valueN1. MP-CLK signal shown in FIG. 5(F) is outputted from calculating &comparing section 306 to motor drive circuit 37Y. In FIG. 5(F), period(t6-t2) is a clock cycle of MP-CLK signal before magnificationcorrection control. At the point in time when counter circuit 43M countsN1/2, MP-CLK signal is reversed from a high level to a low level.

In this example, when Al represents a phase difference between risingtime t1 of YP-CLK signal shown in FIG. 5(C), namely, a count base pointof counter circuit 43Y for Y-color image forming and rising time t2 ofMC-CLK signal, namely, a count base point of counter circuit 43M forM-color image forming, phase detection circuit 305 detects this phasedifference A1.

Further, when P1 represents a phase difference between rising time t3 ofYIDX signal shown in FIG. 5(A) and rising time t4 of MIDX signal shownin FIG. 5(D), phase detection circuit 304 detects this phase differenceP1. In the mean time, E1 represents a rising edge of MP-CLK signal forM-color image forming before magnification correction control shown inFIG. 5(E). In the present example, E1 equals 1.

Each of FIGS. 6(A)-6(F) is a time chart showing an example of operations(YP-CLK signal basis time) after magnification correction control inimage forming section 60. In this example, there is shown the stateafter magnification correction control in the occasion where YP-CLKsignal is a basis (CNTPRD Y=CNTPRD M=N1).

YIDX signals shown in FIG. 6(A) are outputted from index sensor 38Yshown in FIG. 4 after magnification correction control to phasedetection circuits 301 and 304. Y-CNT signal shown in FIG. 6(B) isoutputted from counter circuit 43Y shown in FIG. 4 to calculating &comparing section 303. In FIG. 6(B), a counter cycle is set by Y-CNTPRDsignal to output value N2.

YP-CLK signal shown in FIG. 6(C) is outputted from calculating &comparing section 303 shown in FIG. 4 to motor drive circuit 37Y. InFIG. 6(C), period (t14-t11) is a clock cycle of YP-CLK signal aftermagnification correction control. At the point in time when countercircuit 43Y counts N2/2, YP-CLK signal is reversed from a high level toa low level.

MIDX signal shown in FIG. 6(D) is outputted from index sensor 38M shownin FIG. 4 after magnification correction control to phase detectioncircuit 304. M-CNT signal shown in FIG. 6(E) is outputted from countercircuit 43M shown in FIG. 4 to calculating & comparing section 306. InFIG. 6 (E), a counter cycle is set by M-CNTPRD signal to output valueN2. MP-CLK signal shown in FIG. 6(F) is outputted from calculating &comparing section 306 to motor drive circuit 37Y. In FIG. 6(E), a clockof MP-CLK signal shown in FIG. 6(C) is caused to rise at E2.

In this example, when A2 represents a phase difference between risingtime t11 of YP-CLK signal shown in FIG. 6(C), namely, a count base pointof counter circuit 43Y for Y-color image forming and rising time t13 ofMC-CLK signal, namely, a count base point of counter circuit 43M forM-color image forming, phase detection circuit 305 detects this phasedifference A2.

When N1 represents a counter output value of MP-CLK signal beforemagnification correction control, N2 represents a counter output valueof MP-CLK signal after magnification correction control, AP representsan amount of phase adjustment of polygon mirror 42M and P2 represents aphase difference between YIDX signal after magnification correctioncontrol and MIDX signal for M-color image forming, calculating &comparing section 306 carries out operation of the following expression(1).P2=(P1+AP)×N2/N1   (1)

A counter base point for M-color image forming after magnificationcorrection control shown in FIG. 6(E) is represented by E2. Togetherwith this, the calculating & comparing section 306 carries out anoperation for the following expression (2), when A1 represents a phasedifference between a count base point of counter circuit 43Y for Y-colorimage forming before magnification correction control and a count basepoint of counter circuit 43M for M-color image forming, A2 represents aphase difference between a count base point of counter circuit 43Y forY-color image forming after magnification correction control and a countbase point of counter circuit 43M for M-color image forming, E1represents a counter base point for M-color image forming beforemagnification correction control and E2 represents a rising edge ofMP-CLK signal for M-color image forming for the succeeding page aftermagnification correction control.E2=(A2−A1)+(P2−P1)+E1   (2)By controlling drive of a polygon motor through this operation, it ispossible to execute rotation speed control and phase control of polygonmirror 42Y simultaneously, and thereby to reduce a stabilizing time forrotation.

Incidentally, both C unit 3C and K unit 3K employ the same configurationas that of M unit 3M. CLK1 signals are supplied commonly to countercircuits 43Y, 43M, 43C and 43K respectively for Y-color, M-color,C-color and K-color image forming. Since the same operation is carriedout also between counter circuit 43Y, 43C or 43K for other color imageforming, an explanation will be omitted.

With regard to a rising edge position and a falling edge position foreach of YP-CLK signal, MP-CLK signal, CP-CLK signal and KP-CLK signal,it is possible to determine output timing by deciding a counter value,by comparing counter circuits 43Y and 43M for generating YP-CLK signal,MP-CLK signal, CP-CLK signal and KP-CLK signal. Therefore, CPU 55 canexecute speed control and phase control simultaneously without comparingphases of YIDX signal, MIDX signal, CIDX signal and KIDX signal newly,which can restrain a decline of productivity.

Each of FIGS. 7(A)-7(H) is a time chart showing an example of operations(Y-color basis) after magnification correction control of color copier100.

The assumption of the present example is an occasion wherein Y-colorimage forming processing is executed on a sheet fed out of tray 2 afterimage forming processing on a sheet coming from tray 1 has beencompleted entirely. In this case, image forming processing on a sheetcoming from tray 1 is made to be the state before magnificationcorrection control, Y-color image forming processing on a sheet fed outof tray 2 is made to be-the state after magnification correctioncontrol. In the state before magnification correction control, in otherwords, in the state of giving no phase control amount AP, phasedifference A1 between a count base point of counter circuit 43Y forY-color image forming and a count base point of counter circuit 43M forM-color image forming is detected, and phase difference P1 betweenrising time of UIDX signal and rising time of MIDX signal is detected.

(Before Magnification Correction Control)

Under the foregoing serving as operation conditions, VTOP signal shownin FIG. 7(A) rises at time T21′ when a leading edge of a sheet fed outof tray 1 is detected and the VTOP signal is synchronized with YIDXsignal shown in FIG. 7 (D). YVV start timing shown in FIG. 7(B) rises attime T22′ when an unillustrated YIDX counter is started, the number ofpulses of YIDX signal is counted, and the YVV start timing issynchronized with the YIDX signal. YTV signal shown in FIG. 7(C) risesat time T23′ when the Y V signal is synchronized with YIDX signal shownin FIG. 7(D). Y-color image forming is carried out on a sheet comingfrom tray 1 during the period of “H” level of the YVV signal.

In this case, YIDX signals shown in FIG. 5(A) are outputted to phasedetection circuits 301 and 304 from index sensor 38Y shown in FIG. 4before magnification correction control. Y-CNT signal shown in FIG. 5(B)is outputted to calculating & comparing section 303 from counter circuit43Y shown in FIG. 4. In this case, a counter cycle shown in FIG. 5(B) isset to output value N1 by Y-CNTPRD signal. Further, YP-CLK signal shownin FIG. 5(C) is outputted from calculating & comparing section 303 tomotor drive circuit 37Y. In the example shown in FIG. 5(C), period(t5-t1) is a clock cycle of YP-CLK signal before magnificationcorrection control.

M-color image forming on a sheet coming from tray 1 is carried outduring the period of “H” level of the MVV signal shown in FIG. 7(E). Inthis case, MIDX signal before magnification correction control shown inFIG. 5(D) is outputted to phase detection circuits 304 from index sensor38Y shown in FIG. 4. M-CNT signal shown in FIG. 5(E) is outputted tocalculating & comparing section 306 from counter circuit 43M shown inFIG. 4. Further, a counter cycle shown in FIG. 5(E) is set to outputvalue N1 by M-CNTPRD signal. MP-CLK signal shown in FIG. 5(F) isoutputted from calculating & comparing section 306 to motor drivecircuit 37Y. In the example shown in FIG. 5(F), period (t6-t2) is aclock cycle of MP-CLK signal before magnification correction control.

C-color image forming on a sheet coming from tray 1 is carried outduring the period of “H” level of the CVV signal shown in FIG. 7(F). KVVstart timing shown in FIG. 7(G) rises at time T24′ when an unillustratedKIDX counter is started, the number of pulses of YIDX signal is counted,and the KVV start timing is synchronized with the YIDX signal. KVVsignal shown in FIG. 7(H) rises at time T25′ when the KVV signal issynchronized with KIDX signal shown in FIG. 7 (I). K-color image formingis started at “H” level of the KVV signal, and K-color image forming iscarried out on a sheet coming from tray 1 during the period of the “H”level.

(After Magnification Correction Control)

In the image forming unit 3Y wherein Y-color image forming on a sheetcoming from tray 1 has been completed, rotation speed control for thepolygon mirror for Y-color image forming is conducted for executingmagnification correction control for image forming on the succeedingpage rotation speed control for the polygon mirror for Y-color imageforming is executed after KVV signal shown in FIG. 7 (H) rises. Thereason for this is that image forming start timing for each color iscreated with a basis of Y-color. In the course of this control for speedchange, a frequency of YIDX signal is fluctuated.

In this example, YIDX signals shown in FIG. 6(A) after magnificationcorrection control are outputted from index sensor 38Y shown in FIG. 4to phase detection circuits 301 and 304. Y-CNT signal shown in FIG. 6(B)is outputted from counter circuit 43Y shown in FIG. 4 to calculating &comparing section 303.

In the example shown in FIG. 6(B), a counter cycle is set to outputvalue N2 by Y-CNTPRD signal. YP-CLK signal shown in FIG. 6(C) isoutputted from calculating & comparing section 303 to motor drivecircuit 37Y. In the example shown in FIG. 6(C), period (t14-t11) is aclock cycle of YP-CLK signal after magnification correction control. Inthis example, Y-color image forming processing on a sheet fed out oftray 2 after waiting for stabilizing time Ty that is required forpolygon mirror 42Y to be stabilized in terms of rotation is started,after a rotation speed of polygon motor 36Y is changed.

In this example, M-CNT signal shown in FIG. 6(E), for example, isoutputted from counter circuit 43M shown in FIG. 4 to calculating &comparing section 306, after completion of Y-color image forming (or inthe course of M-color image forming) on a sheet coming from tray 1. InFIG. 6(E), a counter cycle is set by M-CNTPRD signal to output value N2.Further, phase control amount AP is established by M-PHASE signal.

Phase detection circuit 305 detects phase difference Al between risingtime t1 of YP-CLK signal shown in FIG. 5(C), namely, a count base pointof counter circuit 43Y for Y-color image forming, and rising time t2 ofMP-CLK signal, namely, a count base point of counter circuit 43M forM-color image forming. Further, phase difference P1 between rising timet3 of YIDX signal shown in FIG. 5(A), and rising time t4 of MIDX signalshown in FIG. 5(D) is detected by phase detection circuit 304. In themean time, the expression of E=0 holds for a rising edge of MP-CLKsignal for M-color image forming before magnification correction controlshown in FIG. 5(E).

In this case, calculating & comparing section 306 inputs counter outputvalue N1 of MP-CLK signal before magnification correction control,counter output value N2 of MP-CLK signal after magnification correctioncontrol, phase control amount AP of polygon mirror 42M, and phasedifference P1 between rising time of YIDX signal and rising time of MIDXsignal, then, carries out an operation for the expression (1) explainedearlier, and calculates phase difference P2 between YIDX signal aftermagnification correction control and MIDX signal for M-color imageforming.

Then, MIDX signal after magnification correction control shown in FIG.6(D) is outputted from index sensor 38M shown in FIG. 4 to phasedetection circuit 304. M-CNT signal shown in FIG. 6(E) is outputted fromcounter circuit 43M to calculating & comparing section 306 both areshown in FIG. 4. In the example shown in FIG. 6(E) a counter cycle isset by M-CNTPRD signal to output value N2. MP-CLK signal shown in FIG.6(F) is outputted from calculating & comparing section 306 to motordrive circuit 37M.

In this example, phase detection circuit 305 detects phase difference A2between rising time t11 of YP-CLK signal shown in FIG. 6(C), namely, acount base point of counter circuit 43Y for Y-color image forming, andrising time t13 of MP-CLK signal, namely, a count base point of countercircuit 43M for M-color image forming.

In the example shown in FIG. 6(E), a clock of MP-CLK signal aftermagnification correction control rises at E2. In this case, calculating& comparing section 306 inputs phase difference A1 between a count basepoint of counter circuit 43Y for Y-color image forming beforemagnification correction control and a count base point of countercircuit 43M for M-color image forming, phase difference A2 between acount base point of counter circuit 43Y for Y-color image forming aftermagnification correction control and a count base point of countercircuit 43M for M, phase difference P1 between rising time of YIDXsignal and rising time of MIDX signal, phase difference P2 between YIDXsignal after magnification correction control calculated by phasecontrol amount AP and MIDX signal for M-color image forming, and counterbase point E1 for M-color image forming before magnification correctioncontrol, and carries out an operation for expression (2) to calculatecount base point E2 of counter circuit 43M for the M-color image formingon a sheet coming from tray 2 after magnification correction control.

Based on this count base point E2, a rotation speed of polygon motor 36Mis changed. Even in this example, M-color image forming processing isstarted on a sheet fed out of tray 2, after waiting for stabilizing timeTm during which a rotation of polygon mirror 42M is stabilized, from themoment of rotation speed changes for polygon motor 36M and of phasechanges for polygon mirror 42M.

Incidentally, after completion of C-color image forming on a sheetcoming from tray 1, rotation speed changes and phase changes for thepolygon mirror for C-color image forming are controlled, in the same wayas in the M-color image forming mentioned above. In this example,C-color image forming processing on a sheet fed out of tray 2 isstarted, after waiting for stabilizing time Tc during which a rotationof polygon mirror 42C is stabilized, from the moment of rotation speedchanges for polygon motor 36C and of phase changes for polygon mirror42C.

Further, after completion of K-color image forming on a sheet comingfrom tray 1, rotation speed changes and phase changes for the polygonmirror for K-color image forming are controlled. A frequency of KIDXsignal is fluctuated in the course of controlling speed changes andphase changes. In this example, K-color image forming processing on asheet fed out of tray 2 is started after waiting for stabilizing time Tkduring which a rotation of polygon mirror 42K is stabilized, from themoment of rotation speed changes for polygon motor 36K and of phasechanges for polygon mirror 42K.

In the example of image forming operations in the course of switchingtrays of this kind, there is a merit that Y-color image formingprocessing on a sheet fed out of tray 2 can be started before imageprocessing on a sheet coming from tray 1 is totally completed.

In the color copier 100 in the first example, polygon mirrors 42Y-42Kfor respective colors are provided independently of others as statedabove. When Y-color image forming serves as a basis under the assumptionof the foregoing, calculating & comparing section 306 inputs phasedifference A1 between a count base point of counter circuit 43Y beforemagnification correction control and a count base point of countercircuit 43M, phase difference A2 between a count base point of countercircuit 43Y after magnification correction control for Y-color imageforming after magnification correction control and a count base point ofcounter circuit 43M, phase difference P1 between rising time of YIDXsignal and rising time of MIDX signal, phase difference P2 between YIDXsignal after magnification correction control calculated by phasecontrol amount AP and MIDX signal for M-color image forming, and counterbase point E1 before magnification correction control, and carries outan operation for expression (2) to calculate count base point E2 ofcounter circuit 43M for the M-color image forming on a sheet coming fromtray 2 after magnification correction control.

It is therefore possible to shorten stabilizing time that stabilizes arotation for each of polygon mirrors 42Y-42K, compared with aconventional method, because speed control and phase control can beexecuted simultaneously for polygon mirror 42M, polygon mirror 42C andfurther for polygon mirror 42K. Owing to this, a period of time requiredfor magnification changes can be reduced sharply, and thereby, a declineof productivity of operations for magnification correction control canbe restrained, which greatly contributes to continuous high speedprocessing for color images. In other words, even when executingoperations for magnification correction control, the same productivityas in the occasion of executing no operations for magnificationcorrection control can be secured, because image forming on a succeedingpage can be started after waiting for only about a half of stabilizingtime in a conventional method from a termination of image forming on thepresent page.

In this example, CPU 55 executes phase control under the basis ofcounter circuit 43Y of image forming unit 10Y for image forming ofY-color representing the first color for image forming. In this way, aperiod of stabilizing time can be reduced, and unwasteful control can bemade possible.

In this example, when image writing units are arranged in the order ofimage writing unit 3Y for Y-color, image writing unit 3M for M-color,image writing unit 3C for C-color and image writing unit 3K for K-color,wherein the one that forms an image earliest comes first, the CPU 55controls rotations and phases of respective polygon mirrors 42Y, 42M,42C and 42K so that M-color image forming unit 3M may use a base pointof a count cycle of polygon drive CLK generating counter circuit 43Y forpolygon mirror 42Y of Y-color image writing unit 3Y as a basis, C-colorimage writing unit 3C may use a base point of a count cycle of polygondrive CLK generating counter circuit 43M for polygon mirror 42M ofM-color image writing unit 3M as a basis, and K-color image writing unit3K may use a base point of a count cycle of polygon drive CLK generatingcounter circuit 43C for polygon mirror 42C of C-color image writing unit3C as a basis. By doing this, timing restriction can be restrained evenwhen a scale of a machine is large, resulting in control whereinstabilizing time for stabilization in rotation of a polygon mirror isreduced.

Embodiment 2

FIG. 8 is a block diagram showing an example of configuration of colorcopier 200 as the second embodiment.

Unlike the first embodiment, this embodiment is equipped with a pseudoindex signal generating circuit (hereinafter referred to as pseudo IDXgenerating circuit 12), and based on pseudo index signals (main scanningbasis signals), control of rotation speed change and control of phasechange both for a polygon mirror in each color image forming before andafter magnification correction are executed simultaneously (third imageforming apparatus). The pseudo index signal in this case (hereinafterreferred to as MST-IDX signal) means a signal that is created throughcycle establishment based on a cycle of main scanning basis signal(index signal which is called IDX signal hereafter) for drive control ofa polygon mirror.

FIG. 8 is a block diagram showing an example of configuration of acontrol system of color copier 200. The color copier 200 shown in FIG. 8has controller 15′ that determines start timing of image forming on aprescribed surface of sheet P based on MST-IDX signal. To thiscontroller 15′, there are connected pseudo IDX generating circuit 12,image memory 13, image processing section 16, communication section 19,sheet feeding section 20, operation panel 48, image forming section 60′and image reading unit 102.

The controller 15′ has therein ROM 53, RAM 54 for work and CPU 55′. TheCPU 55′ executes color image forming control on a prescribed surface ofsheet P based on IDX signal whose cycle is fluctuated by rotation speedcontrol and phase control of polygon mirror 42Y and on MST-IDX signalhaving a fixed cycle, when forming a dolor image on prescribed sheet P.In this example, the CPU 55′ determines VTOP signal in color imageforming processing from the front face to the rear face of sheet P andVTOP signal in color image forming processing in switching of sheetfeeding from tray 1 to tray 2, based on single MST-IDX signal.

Image forming section 60′ has image writing units 3Y, 3M, 3C and 3Krespectively for Y-, M-, C- and K-color image forming, and inputs imagedata Dy, Dm, Dc and Dk for Y-, M-, C- and K-color image forming fromimage memory for Y-, M-, C- and K-color image forming to operate forforming an image on a prescribed surface of sheet P, based on IDX signalfor Y-, M-, C- and K-color image forming and MST-IDX signal.

Further, the controller 15′ is connected to pseudo IDX generatingcircuit 12 which creates MST-IDX signal that serves as a basis signal inthe case of color image forming. Incidentally, YIDX signal or the likeis a signal whose cycle is fluctuated by rotation speed control andphase control of polygon mirror 42Y, while, MST-IDX signal is one whichis not affected by cycle fluctuation of a polygon mirror, to be set to afixed cycle.

Based on a single MST-IDX signal, the controller 15′ determines imageforming start trigger (VTOP) signal in color image forming processingfrom the front face to the rear face of sheet P and VTOP signal in colorimage forming processing in the case of switching sheet feeding fromtray 1 to tray 2. The controller 15′ executes color image formingcontrol on a prescribed surface of sheet P based on MST-IDX signalcreated by pseudo IDX generating circuit 12 and on IDX signals for Y-,M-, C- and K-color.

Owing to the foregoing, when forming a color image on each of the frontface and the rear face of the sheet, for example, it is possible toexecute accurately magnification correction control on the front faceand the rear face of the sheet P, even when the sheet P shrinks afterimage forming on the front face. Further, when forming color images byswitching sheet feeding from tray 1 to tray 2, it is possible to executeaccurately magnification correction control on different sheets, evenwhen a sheet type in tray 1 is different from that in tray 2. By usingthis MST-IDX signal, a period of time required for changing rotationspeeds of a polygon mirror can be shortened, and the maximumproductivity can be secured independently of a machine size.

Crystal oscillator 11 is connected to pseudo IDX generating circuit 12,and CLK1 signals are generated to be outputted to the pseudo IDXgenerating circuit 12 and to image writing units 3Y′, 3M′, 3C′ and 3K′for Y-, M-, C- and K-color image forming. Incidentally, those having thenames and symbols which are the same as those in the first embodimenthave the same functions, thus, explanation for them will be omitted.

FIG. 9 is a block diagram showing an example of configuration of imagewriting unit 3Y′ for Y-color image forming shown in FIG. 8 and itsperipheral circuit. Image writing unit 3Y′ for Y-color image formingshown in FIG. 9 is connected to crystal oscillator 11, pseudo IDXgenerating circuit 12 and CPU 55′. The image writing unit 3Y′ iscomposed, for example, of crystal oscillator 31, pixel CLK generatingcircuit 32, horizontal synchronizing circuit 33, PWM signal generatingcircuit 34, laser (LD) drive circuit 35, polygon motor 36Y, motor drivecircuit 37Y, index sensor 38Y, polygon drive CLK generating circuit 39,timing signal generator 40′ and Y-VV (Valid) generating circuit 41Y.

In this example, CPU 55′ of the controller 15′ executes phase control onthe basis of an output value of IDX counter circuit 401 establishing acycle of MST-IDX signal. For example, the CPU 55′ outputs Y-PHASE signalrepresenting a phase control value to polygon drive CLK generatingcircuit 39Y, based on a sequence program. The Y-PHASE signal isestablished before the start of phase control of polygon mirror 42Y.

Further, the CPU 55′ outputs equally an image leading edge signal(hereinafter referred to as VTOP signal) to timing signal generator 40′based on the sequence program. The VTOP signal is a signal forsynchronizing conveyance timing for sheet P with image forming timing.

Under the arrangement mentioned above, a frequency of YP-CLK signal tobe supplied to polygon motor 36Y for Y-color image forming can becontrolled by CPU 55′ independently for each of other image formingunits 10M, 10 C and 10K for M-, C- and K-color image forming.

Timing signal generator 40′ for determining image forming start timingfor Y-color image forming is connected to the aforesaid pseudo IDXgenerating circuit 12. The timing signal generator 40′ is furtherconnected to CPU 55′, and selects MST-IDX signal outputted from thepseudo IDX generating circuit 12 based on VTOP signal outputted from CPU55′, when forming an image on the front face, for example, and countsthe number of pulses of MST-IDX signal to determine an image formingstart timing for Y-color image forming on the front face of the sheetbased on the number of counted pulses. Concurrently with determinationof the image forming start timing for Y-color image forming, imageforming start signal (hereinafter referred to as STT signal) isoutputted to Y-VV creating circuit 41Y.

Y-VV creating circuit 41Y counts the number of pulses of YIDX signalbased on STT signal outputted from timing signal generator 40′, andcreates sub-scanning effective area signal (hereinafter referred to asYVV signal) for Y-color image forming on the front face of the sheetbased on the number of counted pulses. YVV signal is outputted to imagememory 83 for Y-color image forming.

Further, timing signal generator 40′ selects MST-IDX signal outputtedfrom pseudo IDX generating circuit 12 based on VTOP signal outputtedfrom CPU 55′ immediately before the start of image forming on the rearface, for example, and counts the number of pulses of the MST-IDX signalto determine image forming start timing for Y-color image forming on therear face of the sheet based on the number of counted pulses.Concurrently with determination of the image forming start timing forY-color image forming, STT signal (image forming start signal) isoutputted to Y-VV creating circuit 41Y.

Y-VV creating circuit 41Y counts the number of pulses of YIDX signalbased on STT signal outputted from timing signal generator 40′, andcreates YVV signal for Y-color image forming on the rear face of thesheet based on the number of counted pulses. YVV signal is outputted toimage memory 83 for Y-color image forming.

Polygon drive CLK generating circuit 39Y is connected to crystaloscillator 11, pseudo IDX generating circuit 12 and to CPU 55′, tooperate to create polygon drive clock signal (YP-CLK signal) based onYIDX signal, CLK1 signal, MST-IDX signal and Y-CNTPRD signal.

The Y-CNTPRD signal is outputted to polygon drive CLK generating circuit39Y from CPU 55′ when forming images on the front and rear faces. TheYIDX signal is outputted to polygon drive CLK generating circuit 39Yfrom index sensor 38Y. The CLK1 signal is outputted to polygon drive CLKgenerating circuit 39Y from crystal oscillator 11. The MST-IDX signal isoutputted to polygon drive CLK generating circuit 39Y from pseudo IDXgenerating circuit 12. An example of internal configuration of polygondrive CLK generating circuit 39Y will be explained, referring to FIG.10. In the mean time, since other image writing units 3M′, 3C′ and 3K′for color image forming also have the same configurations and functions,explanation for them is omitted.

Although crystal oscillator 31, pixel CLK generating circuit 32,horizontal synchronizing circuit 33, PWM signal generating circuit 34,polygon drive CLK generating circuit 39Y, timing generator 40′ and Y-VVgenerating circuit 41Y are included in the image writing unit 3Y′ inthis example, the invention is not limited to this, and these circuitelements may also be included in image processing section 16 or incontroller 15′.

With the controller 15′ constituted in the aforesaid manner, control forrotation speed change and phase change of polygon mirror 42Y is executedafter completion of each color image forming based on MST-IDX signalestablished to prescribed cycle. Due to this, it is possible to executecontrol of rotation speed change and phase change of a polygon mirrorfor that color image forming. Incidentally, those having the names andsymbols which are the same as those in the first embodiment have thesame functions, thus, explanation for them will be omitted.

FIG. 10 is a block diagram showing an example of configuration of apolygon mirror drive system including a pseudo IDX generating circuit.

In this example, pseudo IDX generating circuit 12 is provided, andMST-IDX signals are supplied to units 3Y′-3K′ respectively for Y, M, Cand K to control polygon mirrors 42Y-42K, which is different from thepolygon mirror drive system shown in FIG. 4.

Pseudo IDX generating circuit 12 shown in FIG. 10 is composed of IDXcounter circuit 401 and comparator 402. The IDX counter circuit 401 isconnected to crystal oscillator 11 and to CPU 55′, and counts CLK1signals based on I-CNTPRD signal to output I-CNT signal (output value)to the comparator 402. The I-CNTPRD signal is one to set a cycle ofMST-IDX signal, and it is set to IDX counter circuit 401 from the CPU55′.

The comparator 402 compares I-CNT signal outputted from IDX countercircuit 401 with W-MASTIDX signal for determining a period of a high (H)level of MST-IDX signal outputted from CPU 55′, and outputs MST-IDXsignal. The MST-IDX signals are outputted to units 3Y′-3K′ respectivelyfor Y, M, C and K. Y unit 3Y′ is composed of counter circuit 43Y,polygon drive CLK generating circuit 39Y, motor drive circuit 37Y,polygon motor 36Y and index sensor 38Y.

Counter circuit 43Y determines output timing of YP-CLK signal fordriving polygon motor 36Y (polygon mirror 42Y). When MST-IDX signal is abasis in the case of color image forming, CPU 55′ controls output timingof YP-CLK signal for the next page based on output value Y-CNT signal ofcounter circuit 43Y, a phase difference between MST-IDX signal and YIDXsignal, a phase difference between a base point of count cycle of I-ORGsignal by counter circuit 401 and a base point of a count cycle of Y-ORGsignal by counter circuit 43Y and on Y-PHASE signal showing an amount ofphase control of polygon mirror 42Y.

The CPU 55′ controls a count cycle-established independently of polygonmirrors 42Y-42K shown in FIG. 1 through units 3Y′-3K′ respectively forY, M, C and K, based on MST=IDX signal. In the case of driving polygonmotor 36Y based on YP-CLK signal, the CPU 55′ controls units 3Y′-3K′respectively for Y, M, C and K so that count cycles may be the same eachother concerning image forming on the same page, and establishes countcycles individually for units 3Y′-3K′ respectively for Y, M, C and K toexecute speed control.

Further, the CPU 55′ controls calculating & comparing section 303 tocalculate an amount of phase control based on a base point of a countcycle of I-ORG signal by IDX counter circuit 401, a base point of acount cycle by counter circuit 43Y on which the aforesaid speed controlhas been completed, and on an amount of phase control (an amount ofadjustment for phase difference deviation), to execute phase control forcontrolling a phase of YP-CLK signal based on this amount of phaseadjustment.

Polygon drive CLK generating circuit 39Y is connected to counter circuit43Y to generate YP-CLK signal, referring to an output value of thecounter circuit 43Y. The polygon drive CLK generating circuit 39Y hasphase detecting circuit 301 for indexing, phase detecting circuit 302for a counter and calculating & comparing section 303.

Phase difference PY between MST-IDX signal and YIDX signal is detectedby the phase detecting circuit 301. Phase difference AY between a basepoint of a count cycle of Y-ORG signal by counter circuit 43Y forY-color image forming and a base point of a count cycle of Y-ORG isdetected by the phase detecting circuit 302. The phase detectingcircuits 301 and 302 are connected to calculating & comparing section303 constituting an example of an operation section that carries outoperation for phase difference PY, phase difference AY and Y-PHASE tocalculate an amount of phase adjustment. In this example, an amount ofphase adjustment equaling zero is outputted because of Y-color imageforming basis. Incidentally, an explanation of internal configurationsof units 3M′-3K′ respectively for M, C and K will be omitted here,because each of them employs the same configuration and same function asin unit 3Y′ for Y.

Each of FIGS. 11(A)-11(E) is a time chart showing operation examples (inMST-IDX basis) before magnification correction control in image formingsection 60′. In this example, there is shown the state beforemagnification correction control (CNTPRD I=CNTPRD M=N1) under theoccasion where MST-IDX signal is a basis.

MST-IDX signals shown in FIG. 11(A) are outputted from comparator 402shown in FIG. 10 before magnification correction control to phasedetecting circuit 301 of unit 3Y′ for Y and to phase detecting circuitsof other unillustrated units for M, C and K. I-CNT signal shown in FIG.11(B) is outputted to comparator 402 from IDX counter circuit 401 ofpseudo IDX generating circuit 12 shown in FIG. 10. In FIG. 11(B), acounter cycle is set to output value N1 by I-CNTPRD signal.

YIDX signal shown in FIG. 11(C) is outputted from index sensor 38Y shownin FIG. 10 before magnification correction control to phase detectingcircuit 301. Y-CNT signal shown in FIG. 11(D) is outputted from countercircuit 43Y shown in FIG. 10 to calculating & comparing section 303. InFIG. 11(D), a counter cycle is set to output value N1 by Y-CNTPRDsignal.

YP-CLK signal shown in FIG. 11(E) is outputted from calculating &comparing section 303 shown in FIG. 10 to motor drive circuit 37Y. InFIG. 11(C), a period (t5-t1) is a clock-cycle of YP-CLK signal beforemagnification correction control. YP-CLK signal is reversed from highlevel to low level at the point in time when counter circuit 43Y countsN1/2.

In this example, when P1′ represents a count base point of IDX countercircuit 401 shown in FIG. 11(A), namely, a phase difference betweenrising time t21 of MST-IDX signal and rising time t23 of YIDX signalshown in FIG. 11(C), phase detecting circuit 301 detects this phasedifference P1′.

When A1′ represents a phase difference between rising time t21 ofMST-IDX signal shown in FIG. 11(A), namely, a count base point of IDXcounter circuit 401 and rising time t22 of YP-CLK signal shown in FIG.11(D), namely, a count base point of counter circuit 43Y for Y-colorimage forming, phase detecting circuit 302 detects this phase differenceA1′. Incidentally, a counter base point for Y-color image forming beforemagnification correction control shown in FIG. 11(D) is made to be E1′which equals zero in this example.

Each of FIGS. 12(A)-12(E) is a time chart showing operation examples (inMST-IDX basis) before magnification correction control in image formingsection 60′. In this example, there is shown the state aftermagnification correction control (CNTPRD I=CNTPRD M=N2) under theoccasion where MST-IDX signal is a basis.

MST-IDX signals shown in FIG. 12(A) are outputted from comparator 402shown in FIG. 10 after magnification correction control to phasedetecting circuit 301 of unit 3Y′ for Y and to phase detecting circuitsof other unillustrated units for M, C and K. I-CNT signal shown in FIG.12(B) is outputted to comparator 402 from IDX counter circuit 401 ofpseudo IDX generating circuit 12 shown in FIG. 10. In FIG. 12(B), acounter cycle is set to output value N2 by I-CNTPRD signal.

YIDX signal shown in FIG. 12(C) is outputted from index sensor 38Y shownin FIG. 10 after magnification correction control to phase detectingcircuit 301 and phase detecting circuit 304. Y-CNT signal shown in FIG.12(D) is outputted from counter circuit 43Y shown in FIG. 10 tocalculating & comparing section 303. In FIG. 12(D), a counter cycle isset to output value N2 by Y-CNTPRD signal. YP-CLK signal shown in FIG.12(E) is outputted from calculating & comparing section 303 shown inFIG. 10 to motor drive circuit 37Y.

In this example, when A2′ represents a phase difference between a countbase point (time t31) of IDX counter circuit 401 shown in FIG. 12(B) anda count base point (time t32) of counter circuit 43Y for Y-color imageforming, phase detecting circuit 302 detects this phase difference A2′.

For example, when N1 represents a counter output value of YP-CLK signalbefore magnification correction control, N2 represents a counter outputvalue of YP-CLK signal after magnification correction control, P1′represents a phase difference between rising time t21 of MST-IDX signalshown in FIG. 11(A) and rising time t23 of YIDX signal shown in FIG.11(C), ΔP represents an amount of phase control of polygon mirror 42Yand P2′ represents a phase difference between MST-IDX signal aftermagnification correction control and YIDX signal for Y-color imageforming, the calculating & comparing section 303 carries out operationfor the following expression (3).P2′=(P1′+ΔP)×N2/N1   (3)

In this case, E2 represents a counter base point for Y-color imageforming after magnification correction control shown in FIG. 12(E).Together with calculation of phase difference P2′, the calculating &comparing section 303 carries out operation for the following expression(4), when A1′ represents a phase difference between rising time t21 ofMST-IDX signal shown in FIG. 11(A), namely, a count base point of IDXcounter circuit 401 and rising time t22 of YP-CLK signal shown in FIG.11(D), namely, a count base point of counter circuit 43Y for Y-colorimage forming, A2′ represents a phase difference between a count basepoint of IDX counter circuit 401 shown in FIG. 12(B) (time t31) and acount base point of counter circuit 43Y for Y-color image forming (timet32), E1′ represents a counter base point for Y-color image formingbefore magnification correction control, and E2′ represents a count basepoint of counter circuit 43Y for the Y-color image forming on the nextpage after magnification correction control.E2′=(A2′−A1′)+(P2′−P1′)+E1′  (4)

Incidentally, M unit 3M′, C unit 3C′ and K unit 3K′ employ the sameconfiguration as that of Y unit 3Y′. CLK1 signals are supplied commonlyto counter circuits 43Y, 43M, 43C and 43K respectively for Y-color,M-color, C-color and K-color image forming. Since the same operation iscarried out also between IDX counter circuit 401 and counter circuit43M, 43C or 43K for other color image forming, an explanation will beomitted.

As stated above, it is possible to determine the counter value to beset, by comparing phases between IDX counter circuit 401 and countercircuits 43Y, 43M, 43C and 43K respectively for generating YP-CLKsignal, MP-CLK signal, CP-CLK signal and KP-CLK signal, regardingpositions of edges for rising and falling of YP-CLK signal, MP-CLKsignal, CP-CLK signal and KP-CLK signal. Therefore, CPU 55′ can executespeed control and phase control simultaneously even when a singleMST-IDX signal is made to be a basis. Accordingly, even whenmagnification correction control is involved, a decline of itsproductivity can be restrained.

Next, operations of color copier 200 relating to the second example willbe explained. Each of FIGS. 13(A)-13(M) is a time chart showingoperation examples (MST-IDX signal basis) before and after magnificationcorrection control of the color copier 200.

In this example, what is given as an example is an occasion wherein asingle MST-IDX signal is outputted from pseudo IDX generating circuit 12shown in FIG. 10, and speed control and phase control for polygon mirror42Y and others respectively for Y-color, M-color, C-color and K-colorare executed, under the basis of MST-IDX signal that is fixed to aprescribed cycle (frequency). In this example, MST-IDX signal shown inFIG. 13(L) is used as base index signal in the case of phase adjustingfor a polygon.

Further, with respect to YVV-, MVV-, CVV- and KVV-start timingrespectively for Y-, M-, C- and K-color, there is introduced, as anexample, the occasion wherein the start timing is determined by countingthe number of pulses by using MST-IDX signal as a count source, duringperiod TX from rising time of VTOP signal for image forming on a frontface of tray 1 to the time for rewriting the established value for thefront face of tray 1 to established value for the front face of tray 2.Incidentally, regarding the established value of frequency dividing forMST-IDX, the established value for the front face of tray 1 shown inFIG. 13(M) together with an unillustrated print start is stored in RAM54 or the like. After that, an established value for a front face oftray 1, an established value for a front face of tray 2, an establishedvalue for a rear face of tray 1 and an established value for a rear faceof tray 2 are set in order.

In this example, frequency dividing setting change timing of MST-IDXsignal is determined after the KVV signal for the last color (K-color)has risen and before the start of phase change of the first imageforming color (Y). Further, there is executed sheet feeding control forfeeding out sheet P2 to an image forming system from tray 2, so thatVTOP signal for image forming on a front face of tray 2 may be detectedafter completion of phase change control of the first image formingcolor (Y) for sheet P1 coming from tray 1.

(Example of Control for Front Face Imaging with Tray 1)

VTOP signal for front face imaging with tray 1 shown in FIG. 13(A) risesat time T11 when a leading edge of sheet P1 fed out of tray 1 isdetected. After that, VTOP signal rises, and then, the number of pulsesof MST-IDX signal is counted by an unillustrated MST-IDX counter, and attime T2 that is synchronized with a pulse count output of the firstMST-IDX signal, YVV start timing shown in FIG. 13(B) rises. After that,the YVV start timing falls at time T13 that is synchronized with thesecond pulse count output of the MST-IDX signal.

The number of pulses of YIDX signals shown in FIG. 13 (D) is counted byan unillustrated YIDX counter after generation of YVV start timing, andat time T14 that is synchronized with a pulse count output of the firstYIDX signal, YVV signal for front face of tray 1 shown in FIG. 13 (C)rises. An “H” level period of YVV signal is determined by counting theactual YIDX signal. During this “H” level period of the YVV signal,Y-color image forming processing on the front face of sheet P1 comingfrom tray 1 is carried out. After KVV signal for K-color has beenchanged from “L” level to “H” level after completion of the Y-colorimage forming processing, frequency dividing (ratio) setting of acounter for MST-IDX is changed, and then, control of a rotation speedand a phase for polygon mirror 42Y for Y-color image forming isexecuted. In this example, CPU 55′ executes control of a rotation speedand a phase for polygon mirror 42Y for Y-color based on MST-IDX signal.

For example, in the calculating & comparing section 303 of Y unit 3Y′for which Y-CNTPRED signal and Y-PHASE signal are set from CPU 55′,there are carried out operations for counter output value N1 of YP-CLKsignal before magnification correction control, counter output value N2of YP-CLK signal after magnification correction control, phasedifference P1′ between a count base point of IDX counter circuit 401shown in FIG. 11(A), namely, rising time t21 of MST-IDX signal andrising time t23 of YIDX signal shown in FIG. 11(C) and phase differenceP2′ between MST-IDX signal after magnification correction control basedon the expression (3) by inputting phase control amount ΔP of polygonmirror 42Y and YIDX signal for Y-color image forming.

Together with the foregoing, the calculating & comparing section 303carries out an operation for count base point E2′ of counter circuit 43Yfor the Y-color image forming for the next tray (next page) aftermagnification correction control, based on the expression (4) explainedearlier, by inputting phase difference A1′ between rising time t21 ofMST-IDX signal shown in FIG. 11(A), namely, a count base point of IDXcounter circuit 401 and rising time t22 of YP-CLK signal shown in FIG.11(D), namely, a count base point of counter circuit 43Y for Y-colorimage forming, phase difference A2′ between a count base point (timet31) of IDX counter circuit 401 shown in FIG. 12(B) and a count basepoint (time t32) of counter circuit 43Y for Y-color image forming, andcounter base point E1′ for Y-color image forming before magnificationcorrection control. In this example, Y-color image forming on a sheetfed out of tray 2 is started after waiting for stabilizing time Ty′during which the rotation of polygon mirror 42Y is stabilized, after thechange of rotation speed of polygon motor 36Y.

With respect to MVV start timing shown in FIG. 13(E), the number ofpulses of MST-IDX signal is counted by an unillustrated MST-IDX countereven after generation of YVV start timing, and the MVV start timingrises at a time synchronized with the fourth pulse count output of theMST-IDX signal, and it falls at a time synchronized with the fifth pulsecount output, in this example. With respect to MVV signal for the frontface of tray 1 shown in FIG. 13(F), the number of pulses of MIDX signalis counted by an unillustrated MIDX counter after generation of MVVstart timing, and the MVV signal rises at a time synchronized with apulse count output of the first MIDX signal. A period of “H” level ofthe MVV signal is determined by counting the actual number of pulsesof-MIDX signal. During this “H” level period of the MVV signal, M-colorimage forming processing is carried out on the front face of sheet P1coming from tray 1. After completion of this M-color image formingprocessing, control of rotation speed and phase of polygon mirror 42Mfor M-color image forming is carried out. In the mean time, sinceoperations in M unit 3M′ are the same as those in Y unit 3Y′, itsexplanation will be omitted. In this example, and it falls at a timesynchronized with the fifth pulse count output, in this example, M-colorimage forming on a sheet fed out of tray 2 is started after waiting forstabilizing time Tm′ during which the rotation of polygon mirror 42M isstabilized, after the change of rotation speed of polygon motor 36M.

With respect to CVV start timing shown in FIG. 13(G), the number ofpulses of MST-IDX′ signal is counted by an unillustrated MST-IDX countereven after generation of YVV start timing and MVV start timing, and theCW start timing rises at a time synchronized with the seventh pulsecount output of the MST-IDX signal, and it falls at a time synchronizedwith the eighth pulse count output, in this example. With respect to CVVsignal for the front face of tray 1 shown in FIG. 13(H), the number ofpulses of CIDX signal is counted by an unillustrated CIDX counter aftergeneration of CVV start timing, and the CVV signal rises at a timesynchronized with a pulse count output of the first CIDX signal.

A period of “H” level of the CVV signal is determined by counting theactual number of pulses of CIDX signal. During this “H” level period ofthe CVV signal, C-color image forming processing is carried out on thefront face of sheet P1 coming from tray 1. After completion of thisC-color image forming processing, control of rotation speed and phase ofpolygon mirror 42C for C-color image forming is carried out. In the meantime, since operations in C unit 3C′ are the same as those in Y unit3Y′, its explanation will be omitted. In this example, C-color imageforming on a sheet fed out of tray 2 is started after waiting forstabilizing time Tc′ during which the rotation of polygon mirror 42C isstabilized, after the change of rotation speed of polygon motor 36C.

With respect to KVV start timing shown in FIG. 13(I), the number ofpulses of MST-IDX signal is counted by an unillustrated MST-IDX countereven after generation of YVV start timing, MVV start timing and CVVstart timing, and the KVV start timing rises at a time synchronized withthe tenth pulse count output of the MST-IDX signal, and it falls at atime synchronized with the eleventh pulse count output, in this example.With respect to KVV signal for the front face of tray 1 shown in FIG.13(J), the number of pulses of KIDX signal is counted by anunillustrated KIDX counter after generation of KVV start timing, and theKVV signal rises at a time synchronized with a pulse count output of thefirst KIDX signal. A period of “H” level of the KVV signal is determinedby counting the actual number of pulses of KIDX signal. During this “H”level period of the KVV signal, K-color image forming processing iscarried out on the front face of sheet P1 coming from tray 1.

In this example, after KVV signal for the last color (K-color) rises, anestablished value of frequency dividing for MST-IDX shown in. FIG. 13(M)is rewritten from the established value for the front face of tray 1 tothe established value for the front face of tray 2. This switching ofthe established value is executed before phase change control for inrotation speed and phase control for polygon mirror 42Y for Y-colorimage forming, and with a trigger of rising of KVV signal that rises insynchronization with KIDX signal shown in FIG. 13(K). In this example,K-color image forming on a sheet fed out of tray 2 is started afterwaiting for stabilizing time Tk′ during which the rotation of polygonmirror 42Y is stabilized, after the change of rotation speed of polygonmotor 36Y.

(Example of Control for Front Face Imaging with Tray 2)

Further, after Y-color image forming on the front face of sheet P1coming from tray 1 has been completed, and after control of phase changefor polygon mirror 42Y for Y-color image forming has been completed,sheet P2 is fed out to an image forming system from tray 2 through sheetfeeding control by CPU 55′. A leading edge of the sheet P2 fed out oftray 2 is detected at time T16 shown in FIG. 13(A), and VTOP signal forthe front face of tray 2 rises.

After rising of this VTOP signal, the number of pulses of MST-IDX signalis counted by an unillustrated MST-IDX counter, and YVV start timingsignal for the front face of tray 2 shown in FIG. 13(B) rises at timeT17 that is synchronized with pulse count output of the first MST-IDXsignal. After that, YVV start timing signal falls at time T18 that issynchronized with the second pulse count output of MST-IDX signal.

The number of pulses of YIDX signals shown in FIG. 13 (D) is counted byan unillustrated YIDX counter after generation of YVV start timing, andat time T19 that is synchronized with a pulse count output of the firstYIDX signal, YVV signal for front face of tray 2 shown in FIG. 13 (C)rises. In this example, K-color image forming processing is terminatedafter YVV signal for the front face of tray 2 rises, and then, controlof rotation speed and phase of polygon mirror 42K for K-color imageforming is executed. In the mean time, since operations in K unit 3K′are the same as those in Y unit 3Y′, its explanation will be omitted.

The number of pulses of YIDX signals shown in FIG. 13 (D) is counted byan unillustrated YIDX counter after generation of YVV start timing, andat time T19 that is synchronized with a pulse count output of the firstYIDX signal, YVV signal for front face of tray 2 shown in FIG. 13 (C)rises. A period of “H” level of the YVV signal is determined by countingthe actual number of pulses of YIDX signal. During this “H” level periodof the YVV signal, Y-color image forming processing is carried out onthe front face of sheet P2 coming from tray 2. After completion of thisY-color image forming processing, control of rotation speed and phase ofpolygon mirror 42Y for Y-color image forming is carried out. In the meantime, since operations for the front face of tray 2 of Y unit 3Y′ arethe same as those for the front face of tray 1 of Y unit 3Y′, itsexplanation will be omitted.

In the color copier 200 relating to the second example, pseudo IDXgenerating circuit 12 is provided, and control of a rotation speedchange and control of a phase change for the polygon mirror in eachcolor image forming before and after magnification correction areexecuted simultaneously, based on MST-IDX signal, as stated above. Underthis assumption, when Y-color image forming is a basis, phase differenceP2′ between MST-IDX signal after magnification-correction control andYIDX signal for Y-color image forming is calculated based on expression(3) in calculating & comparing section 303 of unit 3Y′ for Y whereY-CNTPRED signal and Y-PHASE signal are set from CPU 55′. Together withthis, the calculating & comparing section 303 of Y unit 3Y′ is caused tocalculate count base point E2′ of counter circuit 43Y for the Y-colorimage forming after magnification correction control based on theexpression (4) explained earlier. Calculation is carried out in the sameway as in the foregoing even for each of units 3M′-3K′ respectively forM-K.

Therefore, compared with a conventional method, it is possible toshorten a period of stabilizing time during which a rotation for each ofpolygon mirrors 42Y-42K is stabilized, because speed control and phasecontrol can be executed simultaneously for polygon mirror 42Y, polygonmirror 42M, polygon mirror 42C and polygon mirror 42K. Owing to this, adecline of productivity in the case of magnification correction controloperations can be restrained, which greatly contributes to continuoushigh speed processing for color images. In other words, image formingfor next tray 2 can be started after waiting for the stabilizing timethat is about a half of that in the conventional method, aftercompletion of image forming for the tray 1, thus, the same productivityas that in the occasion of no execution of operations for magnificationcorrection control can be secured even when operations for magnificationcorrection control are executed.

Embodiment 3

FIG. 14 is a block diagram showing an example of configuration of acontrol system in color copier 300 relating to the third embodiment.

The color copier 300 shown in FIG. 14 is one to execute color imageforming control on a prescribed surface of sheet P based on two pseudomain scanning basis signals. The color copier 300 is equipped withpseudo IDX generating circuit 12′, image memory 13, controller 15″,image processing section 16, communication section 19, sheet feedingsection 20, operation panel 48, image forming section 60″ and imagereading unit 102 which are all connected to controller 15″.

The pseudo IDX generating circuit 12′ is caused to generate first andsecond MST-IDX signals each of which is a basis signal in the case offorming color images on which a prescribed cycle can be set freely. Thepseudo IDX generating circuit 12′ generates, for example, the firstMST-IDX1 signal having the first cycle and the second MST-IDX2 signalhaving the second cycle that is shorter than the first cycle. ThisMST-IDX1 signal is used to form images on the front face of a sheet andthe MST-IDX2 signal is used to form images on the rear face of a sheet.

The controller 15″ has ROM 53, RAM 54 and CPU 55″. In this example, thepseudo IDX generating circuit 12′ generates MST-IDX1 and MST-IDX2signals, and CPU 55″ determines image forming start timing on the othersurface of sheet P1 or on a surface on one side of the next sheet P2,based on MST-IDX1 signal and MST-IDX2 signal, then, it furtherestablishes cycles of MST-IDX1 signal and MST-IDX2 signal, and executessimultaneously rotation speed change control and phase change control ineach color image forming before and after magnification correction,based on MST-IDX1 signal or MST-IDX2 signal.

The image forming section 60″ is connected to the controller 15″. Theimage forming section 60″ has image writing units 3Y″, 3M″, 3C″ and 3K″respectively for Y-color, M-color, C-color and K-color, and it inputsimage data Dy, Dm, Dc and Dk for Y-color, M-color, C-color and K-colorfrom image memory 13 for Y-color, M-color, C-color and K-color, andoperates to form images on the prescribed surface of sheet P based onMST-IDX1 or MST-IDX signal, selection control signals SS1 and SS2,CNTPRD signal and PHASE signal. Incidentally, those having the names andsymbols which are the same as those in the first and second embodimentshave the same functions, thus, explanation for them will be omitted.

FIG. 15 is a block diagram showing an example of configuration of imagewriting unit 3Y′′ for Y-color image forming extracted from FIG. 14 andits peripheral circuit. The image writing unit 3Y′′ for Y-color imageforming shown in FIG. 15 is connected to crystal oscillator 11, pseudoIDX generating circuit 12′ and CPU 55″.

What is different from the second embodiment is that the pseudo IDXgenerating circuit 12′ generates MST-IDX1 signal and MST-IDX2 signalbased on CLK1 signal. The CLK1 signal is outputted from crystaloscillator 11 to the pseudo IDX generating circuit 12′ and polygon driveCLK generating circuit 39Y.

In this example, CPU 55″ establishes cycles for MST-IDX1 signal andMST-IDX2 signal, and executes phase control with a basis of an outputvalue of an unillustrated IDX counter circuit. For example, the polygondrive CLK generating circuit 39Y is equipped with an unillustratedselector which selects MST-IDX1 signal or MST-IDX2 signal based onselection control signal SS1.

The CPU 55″ outputs the selection control signal SS1 to the polygondrive CLK generating circuit 39Y based on a sequence program. Theselection control signal SS1 is established before the start of planephase control of polygon mirror 42Y. In the same way, the CPU 55″outputs selection control signal SS2 to timing signal generator 40″based on the sequence program. The selection control signal SS2 isgenerated based on a control command that indicates image forming on therear face or tray switching, and is established before an image tipsignal (hereinafter referred to as VTOP signal) rises. The VTOP signalis a signal for synchronizing conveyance timing with image formingtiming for sheet P.

In this example, each of selection control signals SS1 and SS2 showsrespectively selection of the front face or tray 1, for example, at alow level (hereinafter referred to as “L” level), and selection of therear face or tray 2 at a high level (hereinafter referred to as “H”level). By doing this, a frequency of YP-CLK signal to be supplied topolygon motor 36 for Y-color can be controlled by CPU 55″ independentlyof other image forming units 10M, 10C and 10K respectively for M-color,C-color and K-color.

The aforesaid pseudo IDX generating circuit 12 is connected with timingsignal generator 40″ that determines image forming start timing forY-color image forming. When forming images on the front face, the timingsignal generator 40″ selects MST-IDX1 signal or MST-IDX2 signaloutputted from pseudo IDX generating circuit 12′, based on VTOP signaland selection control signal SS2 outputted from CPU 55″, and counts thenumber of pulses of the MST-IDX1 signal or MST-IDX2 signal to determineimage forming start timing for Y-color image forming on the front faceof the sheet, based on the number of counted pulses. Concurrently withthe determination of the image forming start timing for Y-color imageforming, image forming start signal (hereinafter referred to as STTsignal) is outputted to Y-VV creating circuit 41Y.

The Y-VV creating circuit 41Y counts the number of pulses of YIDX signalbased on STT signal outputted from timing signal generator 40″, andcreates sub-scanning effective are signal (hereinafter referred to asYVV signal) based on the number of counted pulses. The YVV signal isoutputted to image memory 83 for Y-color image forming.

Further, the timing signal generator 40″ selects the MST-IDX1 signal orMST-IDX2 signal outputted from pseudo IDX generating circuit 12′, basedon, for example, VTOP signal and selection control signal SS2 outputtedfrom CPU 55″ immediately before image forming on the rear face, andcounts the number of pulses of MST-IDX1 signal or MST-IDX2 signal todetermine image forming start timing for Y-color image forming on therear face of the sheet, based on the number of counted pulses.Concurrently with the determination of the image forming start timingfor Y-color image forming, STT signal (image forming start signal) isoutputted to Y-VV creating circuit 41Y.

The Y-VV creating circuit 41Y counts the number of pulses of YIDX signalbased on STT signal outputted from timing signal generator 40″, andcreates YVV signal for Y-color image forming on the rear face of thesheet based on the number of counted pulses. The YVV signal is outputtedto image memory 83 for Y-color image forming.

Further, polygon drive CLK generating circuit 39Y″ is connected tocrystal oscillator 11, pseudo IDX generating circuit 12 and CPU 55′, andit operates to creates polygon drive clock signal (YP-CLK signal) forY-color image forming, based on YIDX signal, CLK1 signal, MST-IDX signaland Y-CNTPRD signal.

When forming images on both the front face and the rear face, Y-CNTPRDsignal is outputted from CPU 55″ to polygon drive CLK generating circuit39Y. YIDX signal is outputted from index sensor 38Y to polygon drive CLKgenerating circuit 39Y. MST-IDX1 signal and MST-IDX2 signal areoutputted from pseudo IDX generating circuit 12 to polygon drive CLKgenerating circuit 39Y″. FIG. 10 is to be consulted for an example ofinternal configuration of the polygon drive CLK generating circuit 39Y″.In the mean time, since each of other image writing units 3M′, 3C′ and3K′ has also the configuration and function which are the same as thosein the foregoing, descriptions for them will be omitted.

In this example, an explanation has been given by including crystaloscillator 31, pixel CLK generating circuit 32, horizontal synchronizingcircuit 33, PWM signal generating circuit 34, polygon drive CLKgenerating circuit 39Y″, timing generator 40″ and Y-VV generatingcircuit 41Y in the image writing unit 3Y″. However, the invention is notlimited to this, and these circuit elements may also be included inimage processing section 16 or in controller 15″ for the configuration.

Next, an example operations of color copier 300 will be explained. Eachof FIGS. 16(A)-16(O) is a time chart showing operation examples beforeand after magnification correction control of the color copier 300. Inthis example, when switching between the front face and the rear face ofthe sheet for image forming, image forming start timing for the rearface of sheet P is determined based on MST-IDX1 signal and MST-IDX2signal, and rotation speed control and phase control for polygon mirrors42Y-42K for respective colors are executed simultaneously. In that case,rising timing of YP-CLK signal on the rear face of the sheet isdetermined.

In FIG. 16(O), T1 shows a period during which the start timing for eachof YVV signal, MVV signal and CVV signal in the case of image forming onthe front face is determined with MST-IDX1 signal serving as a countsource. Incidentally, a width (W width) of the sub-scanning effectivearea of each of YVV signal, MVV signal and CVV signal in the case ofimage forming on the front face is determined by using actual YIDXsignal, MIDX signal and CIDX signal obtained from index sensor 38Y. As abasis IDX signal in the case of controlling speed and phase of polygonmirror 42Y, MST-IDX1 signal or MST-IDX2 signal is used, one after theother.

Further, in FIG. 16(O), T″ shows a period of time to determine starttiming of each of YVV signal, MVV signal and Cvv signal in the case offorming images on the rear face, with MST-IDX2 signal serving as a countsource. In the mean time, a sub-scanning effective area width (W width)of each of YVV signal, MVV signal and CVV signal in the case of formingimages on the rear face is determined by using actual YIDX signal, MIDXsignal and CIDX signal obtained from index sensor 38Y. MST-IDX2 signalis used as a basis IDX signal in the case of controlling speed and phaseof polygon mirror 42Y.

In this example, color toner images formed on intermediate transfer belt6 are conveyed in the sub-scanning direction in the order of K-color,C-color, M-color and Y-color. Therefore, in the image forming units 10Y,10M, 10C and 10K, images are formed in the order of Y-color, M-color,C-color and K-color. In each of image writing units 3Y″, 3M″, 3C″ and3K″, speed control and phase control are executed under the basis ofpseudo MST-IDX1 signal or MST-IDX2 signal.

With respect to STT signal (image forming start signal) for Y-colorimage forming in the case of forming images on the front face, starttiming for YVV signal is determined by inputting those latched byMST-IDX1 signal in Y-VV generating circuit 41Y of image writing unit3Y″, and by counting them. YVV signal is created by counting YIDX signalof image writing unit 3Y″, with this STT signal (image forming startsignal) for Y-color image forming that serves as a basis. A descriptionwill be given as follows by dividing into three occasions including thecase of image forming on the front face, the case of switching betweenthe front face and the rear face and the case of image forming on therear face.

(Image Forming on the Front Face)

Under these operation conditions, VTOP signal (image tip signal) showingimage forming on the front face in FIG. 16(A) rises in synchronizationwith MST-IDX1 signal at time t1 shown in FIG. 16(N), and these VTOPsignals are outputted from CPU 55″ to timing signal generators 40″ forimage forming of respective colors, Y-VV generating circuit 41Y, M-VVgenerating circuit 41M, C-VV generating circuit 41C and K-VV generatingcircuit 41K.

After that, the number of pulses of MST-IDX1 signal shown in FIG. 16(N)is counted in the timing signal generator 40″, and STT signal forY-color image forming (hereinafter referred to as SST-Y signal) rises attime t2 shown in FIG. 16(D). This STT-Y signal is an image forming startsignal that indicates the start of image forming on the front face forimage forming unit 10Y for Y-color image forming. This STT-Y signalfalls at time t3, and further, the number of pulses of MST-IDX1 signalis counted based on STT-Y signal in Y=VV generating circuit 41Y whichstarts YVV signal at time t4 shown in FIG. 16(E).

For example, in Y-VV generating circuit 41Y, MST-IDX1 signal outputtedfrom pseudo IDX generating circuit 12′ is selected based on VTOP signaloutputted from CPU 55″ and on “L” level selection control signal SS2,and the number of pulses of YIDX signal is counted based on VTOP signalto create YVV signal for Y-color image forming on the front face(sub-scanning effective area signal for Y-color image forming) based onthe number of counted pulses.

YVV signal shown in FIG. 16(E) is outputted to image memory 83 forY-color image forming. In this case, horizontal synchronizing circuit 33shown in FIG. 15 operates to detect horizontal synchronizing signal Shbased on YIDX signal to output to PWM signal generating circuit 34. YIDXsignal shown in FIG. 16(F) is outputted from index sensor 38Y forY-color image forming to horizontal synchronizing circuit 33 and isoutputted to polygon drive CLK generating circuit 39Y″.

The PWM signal generating circuit 34 operates to input horizontalsynchronizing signal Sh and image data Dy for Y-color image forming, andto modulate the image data Dy in terms of pulse width to output laserdrive signal Sy for Y-color image forming to LD drive circuit 35. The LDdrive circuit 35 drives laser diode based on the laser drive signal Syso that laser beam LY for Y-color image forming having prescribedintensity is generated to radiate toward polygon mirror 42Y.

Further, the polygon drive CLK generating circuit 39Y″ creates YP-CLKsignal based on YIDX signal, CLK1 signal, MST-IDX1 signal, MST-IDX2signal, Y-CNTPRD signal and “L” level selection control signal SS1.

Motor drive circuit 37Y drives polygon motor 36Y based on YP-CLK signal.The polygon motor 36Y operates to rotate polygon mirror 42Y. A laserdiode connected to the motor drive circuit 37Y radiates laser beam LY,and the laser beam LY is oscillated by the rotation of polygon mirror42Y for the main scanning for photoreceptor drum 1Y that rotates in thesub-scanning direction. Through this main scanning, an electrostaticlatent image is written on the photoreceptor drum 1Y. The electrostaticlatent image written on the photoreceptor drum 1Y is developed withtoner member for Y-color image forming. A Y-color toner image on thephotoreceptor drum 1Y is transferred onto intermediate transfer belt 6that rotates in the sub-scanning direction (primary transfer).

Then, the number of pulses of MST-IDX1 signal is counted even in thecourse of Y-color image forming, and MVV signal rises at time t5 shownin FIG. 16(H) in order after the image forming start signal for M-colorimage forming shown in FIG. 16(G) (STT-M signal) rises based on MST-IDX1signal, then, CVV signal shown in FIG. 16(J) rises at time t6 after theimage forming start signal for C-color image forming shown in FIG. 16(I)(STT-C signal) rises based on MST-IDX1 signal, and KVV signal rises attime t7 shown in FIG. 16(L) after the image forming start signal forK-color image forming shown in FIG. 16(K) (STT-K signal) rises based onMST-IDX1 signal. The aforesaid processing is carried out even in each ofimage writing units 3M″, 3C″ and 3K″ respectively for M-, C- and K-colorimage forming. When YVV signal falls at time t8 shown in FIG. 16(E)after Y-color image forming has been completed, control for changing arotation speed and a phase is executed in image writing unit 3Y″ basedon YIDX signal shown in FIG. 16(F) for Y-color image forming on the rearface of a sheet.

(Switching Between Front Face and Rear Face)

In this example, CPU 55″ outputs selection control signal SS1 to polygondrive CLK generating circuit 39″ based on a sequence program, andexecutes control of rotation speed and phase of polygon mirror 42Y forY-color based on MST-IDX1 signal or MST-IDX2 signal. For example,Y-CNTPRED signal and Y-PHASE signal are established for image writingunit 3Y″ from CPU 55″. In the image writing unit 3Y″ on which theY-CNTPRED signal and Y-PHASE signal are established, falling of YVVsignal is detected at time t8 shown in FIG. 16(E) and selection controlsignal SS1 is started up to “H” level at time t9 shown in FIG. 16(b).This “H” level selection control signal SS1 is outputted from CPU 55″ topolygon drive CLK generating circuits 39Y″, 39M″, 39C″ and 39K″ forrespective colors, together with frequency control signal Sg.

In the polygon drive CLK generating circuits 39Y″, if the examples shownin FIG. 11 and FIG. 12 are applied, phase difference P2′ betweenMST-IDX1 or MST-IDX2 signal after magnification correction control andYIDX signal for Y-color image forming is calculated based on theaforesaid expression (3) by inputting therein counter output value N1 ofYP-CLK signal before magnification correction control, counter outputvalue N2 of YP-CLK signal after magnification correction control, phasedifference P1′ between a count base point of IDX counter circuit inpseudo index generating circuit 12′, namely, rising time t21 of MST-IDX1signal or MST-IDX2 signal and rising time t23 of YIDX signal for Y-colorimage forming, and phase control amount ΔP of polygon mirror 42Y.

Together with the foregoing, in the polygon drive CLK generatingcircuits 39Y″, count base point E2′ of counter circuit 43Y for theY-color image forming on the rear face of a sheet (next page) beforemagnification correction control is calculated based on the aforesaidexpression (4) by inputting therein phase difference A1′ between risingtime t21 of MST-IDX signal shown in FIG. 11(A), namely, a count basepoint of IDX counter circuit 401 and rising time t22 of YP-CLK signalshown in FIG. 11(D), namely, a count base point of IDX counter circuit401 and a count base point of counter circuit 43Y for Y-color imageforming, phase difference A2′ between a count base point (time t31) ofIDX counter circuit 401 shown in FIG. 12(B) and a count base point (timet32) of counter circuit 43Y for Y-color image forming and counter basepoint E1′ for Y-color image forming before magnification correctioncontrol.

By the calculation stated above, rising timing of YP-CLK signal on therear face of a sheet can be determined. In the polygon drive CLKgenerating circuits 39Y″, YP-CLK signal for forming images on the rearface which has been created and phase-adjusted based on count base pointE2′ of counter circuit 43Y is outputted to polygon motor 36Y. The sameprocessing of calculation is carried out even for each of other imagewriting units 3M″, 3C″ and 3K″.

(Image Forming on the Rear Face)

In this example, in the case of image forming on the rear face of asheet, CPU 55″ starts up a signal of image forming on the rear face of asheet (VTOP signal) based on MST-IDX2 signal, and counts the number ofpulses of MST-IDX2 signal based on the VTOP signal to determine starttiming for image forming on the rear face of a sheet based on the numberof counted pulses. Further, processing of Y-color image forming on therear face of a sheet is started after waiting for stabilizing time Ty″for the rotation of polygon mirror 42Y to be stabilized, from the changeof rotation speed of polygon motor 36Y.

When MVV signal falls after M-color image forming is completed at timet10 shown in FIG. 16(H), rotation speed change and phase change arecontrolled in image writing unit 3M″. In this example, M-color imageforming on the rear face of a sheet is started at time t17 after waitingstabilizing time Tm″ for the rotation of polygon mirror 42M to bestabilized, from the change of rotation speed of polygon motor 36M.

Further, when CVV signal falls after C-color image forming is completedat time t12 shown in FIG. 16(A), rotation speed changes and phasechanges are controlled in image writing unit 3C″. In this example,C-color image forming on the rear face of a sheet is started at time t18after waiting for stabilizing time Tc″ for the rotation of polygonmirror 42C to be stabilized, from the changes of the rotation speed ofpolygon motor 36C.

Further, when KVV signal falls after K-color image forming-is completedat time t16 shown in FIG. 16(L), rotation speed changes and phasechanges are controlled in image writing unit 3K″. In this example,K-color image forming on the rear face of a sheet is started at time t19after waiting for stabilizing time Tk″ for the rotation of polygonmirror 42k to be stabilized, from the changes of the rotation speed ofpolygon motor 36K.

As stated above, color copier 300 relating to the third example isequipped with pseudo IDX generating circuit 12′, and rotation speedchanges and phase changes of a polygon mirror in each color imageforming before and after magnification correction are controlledsimultaneously, based on MST-IDX1 signal and MST-IDX2 signal. Under thisassumption, when Y-color image forming is a basis, phase difference P2′between MST-IDX1 signal or MST-IDX2 signal after magnificationcorrection control and YIDX signal for Y-color image forming iscalculated based on expression (3) in image writing unit 3Y″ whereY-CNTPRED signal and Y-PHASE signal are set from CPU 55″. Together withthis, the image writing unit 3Y″ is caused to calculate count base pointE2′ of counter circuit 43Y for the Y-color image forming aftermagnification correction control based on the expression (4) explainedearlier. Calculation is carried out in the same way as in the foregoing,even for each of image writing units 3M″-3K″.

Therefore, a period of stabilizing time for the rotation of each ofpolygon mirrors 42Y-42K to be stabilized can be shortened, compared witha conventional system, because both speed control and phase control canbe executed simultaneously for each of polygon mirrors 42Y, 42M, 42C and42K owing to this, a decline of productivity of operations formagnification correction control can be restrained even in the case ofapplying MST-IDX1 signal and MST-IDX2 signal, which greatly contributesto continuous high speed processing actions for color images. In otherwords, even when executing operations for magnification correctioncontrol, the same productivity as in the occasion of executing nooperations for magnification correction control can be secured, becauseimage forming for a succeeding tray 2 can be started after waiting for afixed stabilizing time from a termination of image forming for the tray1.

(Possibility of Utilization in the Industrial World)

The present invention can be applied extremely preferably to a black andwhite and color digital multifunctional machine equipped with copyingfunctions, facsimile functions and printer functions and to a copier.

In the first embodiment of the image forming apparatus relating to theinvention, there is provided a controller that executes simultaneouslycontrol for changing rotation speed of the polygonal mirror rotator forchanging image size in the sub-scanning direction and control forcorrecting a correction amount for color registration error depending oncorrection of magnification for image sizes, and for adjusting arotating phase of the polygonal mirror rotator depending on a correctionamount for color registration error after the correction, when formingimages by correcting magnification in terms of image sizes by one pageunit.

Owing to this configuration, it is possible to shorten a stabilizingtime during which the rotation of the polygonal mirror rotator isstabilized, compared with an occasion wherein speed control and phasecontrol of the polygonal mirror rotator are carried out in succession.Due to this, a decline of productivity in correcting operations forimage sizes can be controlled, which contributes greatly to continuoushigh speed processing of color images.

In the second embodiment of the image forming apparatus relating to theinvention, there is provided a controller having a calculating sectionfor correcting image sizes by one page unit, and this calculatingsection calculates a rising edge and a falling edge of drive clocksignals that control a rotation speed of the polygonal mirror rotatorfor the succeeding page based on an amount of phase control calculatedby correcting an amount of correction of color registration errorsdepending on an amount of magnification adjustment, an output value of acounter that is provided independently of each color for; determining acycle of drive clock signal and is controlled independently, a phasedifference between the first main scanning basis signal immediatelybefore conducting magnification correction for image sizes and thesecond main scanning basis signal and on a phase difference between abase point of a count cycle of the counter for generating drive clocksignal of the polygonal mirror rotator and a base point of a count cycleunder the condition of count cycle after correction of magnification forimage size.

Owing to this configuration, it is possible to shorten stabilizing timeduring which a rotation of the polygonal mirror rotator is stabilized,because speed control and phase control of the polygonal mirror rotatorcan be carried out simultaneously, compared with a conventional method.Due to this, a decline of productivity in correcting operations forimage sizes can be controlled, which contributes greatly to continuoushigh speed processing of color images.

In the third embodiment of the image forming apparatus relating to theinvention, there is provided a controller having a calculating sectionfor correcting image sizes by one page unit, and this calculatingsection calculates a rising edge and a falling edge of drive clocksignals that control a rotation speed of the polygonal mirror rotatorfor the succeeding page based on an amount of phase control calculatedby correcting an amount of correction of color registration errorsdepending on an amount of magnification adjustment, an output value of acounter that is provided independently of each color for determining acycle of drive clock signal and is controlled independently, a phasedifference between the first main scanning basis signal immediatelybefore conducting magnification correction for image sizes and thesecond main scanning basis signal and on a phase difference between abase point of a count cycle for generating pseudo index signals and abase point of a counter cycle for generating drive clock signals of thepolygonal mirror rotator for each color unit.

Owing to this configuration, it is possible to shorten stabilizing timeduring which a rotation of the polygonal mirror rotator is stabilized,because speed control and phase control of the polygonal mirror rotatorcan be carried out simultaneously, compared with a conventional method.Due to this, a decline of productivity in correcting operations forimage sizes can be controlled, which contributes greatly to continuoushigh speed processing of color images.

1. An image forming apparatus for forming color images comprising atleast two or more colors, having a function of magnification correctionof image size by one page unit, the image forming apparatus comprising:an image carrier; a polygonal mirror rotator independently provided foreach color; and a controller which simultaneously conducts first controlfor changing rotation speed of the polygonal mirror rotator in order forchanging image size in a sub-scanning direction perpendicular to a mainscanning direction, and second control for correcting a correctionamount for color registration error depending on magnificationcorrection of image size, and for adjusting a rotating phase of thepolygonal mirror rotator depending on the corrected correction amountfor color registration error, where the main scanning direction is adirection in which the image carrier is scanned with an exposure beamcoming from the polygonal mirror rotator.
 2. An image forming apparatusfor continuously forming color images comprising at least two or morecolors, having a function of magnification correction of image size byone page unit, the image forming apparatus comprising: a polygonalmirror rotator which is provided independently for each color imageforming unit; an image carrier on which a latent image is formed by anexposure beam scanned by the polygonal mirror rotator and the latentimage is developed to be a color image; and a controller comprising: acolor registration error detecting section which detects colorregistration error on each color image formed on the image carrier; acolor registration error correcting section which corrects the colorregistration error depending on an amount of the color registrationerror obtained from the color registration error detecting section; anda calculating section which calculates a rising edge and a falling edgeof a drive clock signal controlling a rotation speed of the polygonalmirror rotator for a succeeding page, based on an amount of phasecontrol calculated by correcting an amount of color registration errorcorrection after correction by the color registration error correctingsection depending on an amount of magnification correction, on an outputvalue of a counter provided and independently controlled for each colorto determine a drive clock signal cycle that controls a rotation speedof the polygonal mirror rotator, on a phase difference between a firstmain scanning basis signal generated by detecting an exposure beamscanned by the polygonal rotator for a first color image forming unitimmediately before conducting magnification correction for image sizeswith a sensor arranged in a scanning optical path and the second mainscanning basis signal generated by detecting an exposure beam scanned bythe polygonal rotator for a second color image forming unit with asensor arranged in a scanning optical path, and on a phase differencebetween a first base point of a count cycle of a counter for generatinga drive clock signal of the polygonal mirror rotator for each of thefirst and second color image forming units immediately before conductingmagnification correction for image size and a second base point of acount cycle after the magnification correction for image size, whereinthe controller executes polygonal mirror rotator drive control in a caseof magnification correction for image sizes by the drive clock signal,which controls a rotation speed of the polygonal mirror rotator,generated based on an output of the calculating section..
 3. The imageforming apparatus of claim 2, wherein the controller executes arotational phase control of the polygon mirror rotator with a basis ofthe counter for generating a drive clock signal of the polygonal mirrorrotator for the first color image forming unit.
 4. The image formingapparatus of claim 1, further comprising a first color image formingunit, a second color image forming unit, a third color image formingunit, and a fourth color image forming units, wherein when images areformed in order of the first, second, third and fourth color imageforming units, the controller controls rotational phase for eachpolygonal mirror rotator so that the second color image forming unitapplies a count cycle base point of a counter for generating drive clocksignals of the polygonal mirror rotator of the first color image formingunit as a basis, the third color image forming unit applies a base acount cycle base point of a counter for generating drive clock signalsof the polygonal mirror rotator of the second color image forming unitas a basis, and the fourth color image forming unit applies a countcycle base point of a counter for generating drive clock signals of thepolygonal mirror rotator of the third color image forming unit as abasis.
 5. An image forming apparatus for continuously forming colorimages comprising at least two or more colors, having a function ofmagnification correction of image size for each page, the image formingapparatus comprising: a polygonal mirror rotator which is providedindependently for each color image forming unit; an image carrier onwhich a latent image is formed by an exposure beam scanned by thepolygonal mirror rotator and the latent image is developed to be a colorimage; and a controller comprising: a color registration error detectingsection which detects color registration error on each color imageformed on the image carrier; a color registration error correctingsection which corrects the color registration error depending on anamount of the color registration error obtained from the colorregistration error detecting section; and a calculating section whichcalculates a rising edge and a falling edge of a drive clock signalcontrolling a rotation speed of the polygonal mirror rotator for asucceeding page, based on an amount of phase control calculated bycorrecting an amount of color registration error correction aftercorrection by the color registration error correcting section dependingon an amount of magnification correction, on an output value of acounter provided and independently controlled for each color todetermine a drive clock signal cycle that controls a rotation speed ofthe polygonal mirror rotator, on a phase difference between a first mainscanning basis signal generated by detecting an exposure beam scanned bythe polygonal rotator for a first color image forming unit immediatelybefore conducting magnification correction for image sizes with a sensorarranged in a scanning optical path and the second main scanning basissignal generated by detecting an exposure beam scanned by the polygonalrotator for a second color image forming unit with a sensor arranged ina scanning optical path, and on a phase difference between a base pointof a count cycle for generating a pseudo index signal, which is obtainedby dividing a source oscillation signal of an original oscillator usedin common with generation of drive clock signal of the polygonal mirrorrotator in practicing rotational phase control of the polygonal mirrorrotator so that the pseudo index signal agrees with one plane cycle ofthe polygonal mirror rotator, and a base point of a counter cycle forgenerating drive clock signal of the polygonal mirror rotator for eachcolor unit, wherein the controller executes polygonal mirror rotatordrive control in a case of magnification correction for image sizes bythe drive clock signal, which controls a rotation speed of the polygonalmirror rotator, generated based on an output of the calculating section.6. The image forming apparatus of claim 2, further comprising amagnification correction section to correct an image size by one pageunit, wherein the magnification correction section comprises: a rotationspeed changing section to change the rotation speed of the polygonalmirror rotator according to a magnification adjusting amount in asub-scanning direction; and a pixel clock frequency changing section tochange a pixel clock frequency according to a rotation speed-changingamount of the polygonal mirror rotator and to a magnification adjustingamount in a main-scanning direction.
 7. The image forming apparatus ofclaim 5, further comprising a magnification correction section tocorrect an image size by one page unit, wherein the magnificationcorrection section comprises: a rotation speed changing section tochange the rotation speed of the polygonal mirror rotator according to amagnification adjusting amount in a sub-scanning direction; and a pixelclock frequency changing section to change a pixel clock frequencyaccording to a rotation speed changing amount of the polygonal mirrorrotator and to a magnification adjusting amount in a main-scanningdirection.
 8. The image forming apparatus of claim 1, further comprisinga yellow color image forming unit, a magenta color image forming unit, acyan color image forming unit, and a black color image forming unit. 9.The image forming apparatus of claim 2, further comprising a yellowcolor image forming unit, a magenta color image forming unit, a cyancolor image forming unit, and a black color image forming unit.
 10. Theimage forming apparatus of claim 4, wherein the first, the second, thethird, and the fourth color image forming units are respectively ayellow, a magenta, a cyan, and a black color image forming units. 11.The image forming apparatus of claim 5, further comprising a yellowcolor image forming unit, a magenta color image forming unit, a cyancolor image forming unit, and a black color image forming unit.
 12. Theimage forming apparatus of claim 1, further comprising an intermediatetransfer belt, wherein color images made on the image carrier aretransferred to form superposed color images onto the intermediatetransfer belt, and the superposed color images are collectivelytransferred onto a recording sheet.
 13. The image forming apparatus ofclaim 2, further comprising an intermediate transfer belt, wherein colorimages made on the image carrier are transferred to form superposedcolor images onto the intermediate transfer belt, and the superposedcolor images are collectively transferred onto a recording sheet. 14.The image forming apparatus of claim 4, further comprising anintermediate transfer belt, wherein color images made on the imagecarrier are transferred to form superposed color images onto theintermediate transfer belt, and the superposed color images arecollectively transferred onto a recording sheet.
 15. The image formingapparatus of claim 5, further comprising an intermediate transfer belt,wherein color images made on the image carrier are transferred to formsuperposed color images onto the intermediate transfer belt, and thesuperposed color images are collectively transferred onto a recordingsheet.